(Content separated by topic for ease of reading)

Looking for specific feedback:
* Would you play this?
* What topics, if any, do you want me to clarify or expand upon?
* What parts do you like best?  The worst?
* What should be improved upon or removed?

1. The Setting
Magic and wizards were real in medieval and renaissance times in the sense that these arts were certainly developed and practiced.  Naturally, they didnt have any tangible effects on the real world, but this setting makes it so that magic did have observable effects.  So, in this alternate world, all of the historic events play out generally the same, but many of them are explained by supernatural occurrences; Quetzalcoatl is a real being, there are demons and spirits who may have influenced various outcomes, and perhaps a notoriously mad king or queen like Henry VIII or Juana la Loca was under the effects of a spell.
The setting will provide significant detail for GMs to generate a campaign in say Italy, France, or Germany, (or some fictional equivalent or alternate timeline) while encouraging them to educate themselves for the purpose of campaign building.  My design goal for the game is two-fold: a fun learning tool while also being immersed in a world that operates independently of the players.

Where Events Diverge
Due to there being all sorts of fantastic creatures and rogue warlocks (despite both being extremely rare in every day life - little more than rumors and fearful whispers), a new economic class emerges.  Adventurers, essentially.  Knights who find themselves more and more obsolete on the battlefield find their niche in slaying great beasts with lances and defeating cultists in hand to hand combat.  Opportunistic peasants and burghers crawl through dark corners seeking artifacts to sell or a bounty on a local nuisance, developing their skill in arms along the way. Wizards become abjurers in service to Rome, dispelling curses and banishing fiends.  Strongholds of awesome power rise up in remote locations in the Alps, Scottish Highlands and Carpathians.  Men fall in love with nymphs or to their doom with mermaids.

2. The System
The mechanics seek to emulate combat described in contemporary manuscripts and demonstrated by modern historical martial artists to a high degree of fidelity. In order to do this, I wanted to keep numerical abstractions and bookkeeping out of the process as much as possible. There will be an adjustment period at first with the complexity but it's my hope the quick time-to-resolve, the breadth of meaningful choices, and the limited volume of numerical values will keep conflicts interesting.  Here's an overview:
Only a single six-sided die is used for all conflict resolution, chosen because it's simple and the most commonly found die.  There are two types of die roll: an effect roll and a response roll (notice there is no attack roll vs. difficulty).  And there are two types of actions: an initiated action and a response action.  An initiated action is an action a player deliberately chooses on their turn and a response action is an automatic behavior triggered by an initiated action.  
In order to determine which actions are successful, each action's governing attribute is compared against the other and the highest is successful, where some action types have natural advantages against others.  Governing attributes include "Responsiveness", "Mobility", "Power", and "Defense" (represents your weapon's hand guard or shield you're holding), among others.  Most attributes are generally the same by default (for the sake of sanity) and primarily influenced by the weapons and armor you have equipped; armor increases protection and lowers mobility, helmets lower responsiveness and recovery, weapons increase power and reach, shields increase defense and lower mobility and recovery, and so on.

Player Agency in Tactical Choice
On a player's turn, they can assume a guard or a readied position.  In the general mechanics which applies to all conflict resolution, this is called a "frame".  The frame or guard you are in determines the effectiveness of your initiated action and also determines your range of response behaviors.  The response behavior that actually occurs is dictated by the response die roll.
To make an example, say you have a longsword and you are in The Woman's Guard, which only well-trained characters can use.  You chose this guard because the enemy has a spear and spear thrusts are very difficult to push off-line (a parry response has a disadvantage against any powerful weapon).  Because you're in this guard, a "powerful block" response is highly likely (50% on a d6), although not guaranteed.  In contrast, if you were in a more passive guard, the "powerful block" response would either be unlikely or not possible at all
To make another example, it's your turn and you're in a guard where your sword point is extended.  This gives your feint an attribute advantage if and only if your reach is longer.  Their sword is shorter than yours, so you feint.  If you succeed and roll high on your effect die, your opponent is forced out of his guard and into a lower class "frame", giving you a chance to follow-up with a devastating blow.  If you succeed and roll low on your effect die, the GM will reveal the response behaviors of your opponent's frame

Health and Damage
The generic term for capacity in a conflict is called "composure".  Composure in combat is an amalgamation of physical health, stamina, and mental preparedness.  For easier variable management, each character has the same maximum composure value, although some characters will be able to recover composure more easily or be more resistant to loss of composure. The following events can cause your composure value to change for better or worse: being struck, losing your balance or falling or being thrown, having combat anxiety or being intimidated by a fearsome opponent, using your turn to catch your breath, watching your allies die, being rallied by a knight or simply witnessing a knight in shining armor on your side, winning an exchange, or using a powerful fighting technique
When your composure is low, you cannot get into advanced guards or frames.  When your composure drops to zero or below, your character will either be incapacitated or killed.  A single strike from a lethal weapon always has the potential to incapacitate or kill a character, no matter how strong they are.
 
Circumventing Player Timidity
Because this system can be extremely punishing (one-hit kills; no health potions or resurrection spells; infection), I expect players to avoid conflict as much as possible.  To help push them towards a life of adventure, a luck mechanic is installed, in which points are generated or lost by actions relevant to the character's ideology.  Point expenditure might be triggered by a lethal blow, for instance, where a combat-oriented player character moves out of the way just in time.  Further, since the setting is historical, player characters are encouraged to yield if they are losing, as it was common for victors to value honor (or ransom or avoiding prosecution) over killing their enemies.  This is the time of chivalry, after all.

3. Equipment
Armor
Impact tests of authentic armors and replicas have shown that armor was very effective at keeping combatants alive.  Gambesons can withstand cuts from swords and arrows at distance, mail can stop arrows and spears as well as some crossbow bolts, and plate can stop nearly everything except a heavy lance (even bullets in some cases).
In mechanical terms, armor lowers the effect roll value of a strike by a range of 1-6 (protection attribute).  Certain techniques or weapons will allow a combatant to either inflict concussive or puncturing force with a heavy blow or to get inside gaps in the armor with precise strikes and hardened points (e.g. using a "half-sword" guard).
Unfortunately, armor has a lot of drawbacks.  Putting on a full harness of plate can take about 20 or so minutes with a skilled assistant and is a pain to transport; armor gets really hot; guards may harass you if you're strutting around in war gear; weight on your limbs tires you out (lowered recovery and mobility attribute, especially for cheap munition armor); and it's hard to breathe and see with a visor down (lowered responsiveness attribute).

Weapons
As opposed to common belief, weapons are light and fast, even big ones.  Skilled combatants don't hack or swing with gross shoulder motions, but instead make swift motions primarily with the wrists and elbows and aided by the weapon's distribution of weight.  This allows for a dazzling repertoire of sharply turning cuts, effortless sweeps, complex strike combinations, precise aiming, responsive parries, repeated jabs, ripostes, trapping and disarming an opponent's weapon, and tireless moulinets.  
Mechanically, each weapon will have a unique distribution of attribute modifiers.  These modifiers will determine the situations in which a weapon can be effectively employed and what guards and techniques are best suited for it.  A two-handed sword can singly fend off a group of attackers.  A war sword (longsword) or pollaxe can dominate in dismounted armored combat.  A buckler can protect a combatant while closing the gap.  A bastard sword can be quickly drawn at the hip to assume a guard and used effectively against all types of weapons.  A mace can bash helmets with simple techniques.  Spears have a reach advantage and can be dangerous in untrained hands or tight groups.  With the combination of guards, environmental factors, and weapon attributes, meaningful strategic possibilities become so numerous that large differences in power can be managed with careful thought.  

4. Magic
This is the most complex aspect of play in terms of rules.  Each discipline of magic has its own logic, but I'll try to simplify and condense the concepts as best as I can.

Acquisition
A sorcerer doesn't really *get* spells.  Nor is there a mana pool.  Magic is a pain to craft and requires active role-playing.  Since sorcery is so rare and difficult to acquire and use, it is proportionately powerful.  

Elemental Effects
Elements must be stored in a vessel of some sort (Milanese sorcerers prefer glass polyhedra based on Da Vinci's designs), usually affixed to a staff so you don't do silly things like melt your face off.  "Elements" don't simply mean fire, air, and earth, but rather things like heat, light, phases of matter, and mechanical forces.  Since science and empiricism aren't proper philosophical disciplines yet, many of these elements as well as the arcane arts collectively are understood from an Aristotelian model.  
Elemental energy is granted by spirits allowing you to take from their lands in exchange for some favor, by stealing from the land, or by trapping or merging with the spirit.  A non-destructive (to the land, anyway - you might get struck by lightning) and more powerful method of acquiring elements is to connect leyline nodes with the Firmament in order to create a conduit between heaven and earth - this requires a lot of annoying trigonometry and application of astrology (for the character, not the player!)

Invoking, Conjuring, Evoking, and Channeling
Activating an effect typically requires a special word or incantation.  The words themselves have no power, but are often mnemonics to execute the proper notes, cadence, or harmonics which an effect will respond to based on its behavioral properties; for clarity, try to imagine what kind of sounds and tempo might represent a raging fire or why a tenor is better than a soprano at moving the earth.  In many instances, a chant or prayer came first and the effects were discovered by accident.  Only in late medieval times was a theoretical model developed around this process.  
To conjure a complex spell out of an effect or group of effects (such as a tempest or the illusion of a dinner banquet), a sorcerer might need assistants, mechanical devices, mirrors, a great voice, or they may need to be a trained artist to vividly imagine how the spell should play out.  Shooting out a blast of energy may require an athletic body to control the staff and making the energy move in precise patterns requires a wand and a dexterous hand

Intuitive Magic
Some people just *get it*.  But most likely they bargained with a demon to teach them simple tricks.  Many of these tricks aren't puny, but rather they're easy to perform because a huge price was paid for them.  Demonology is a very dense topic in the setting - too dense for this post- so I'll just leave it at this.

Have you heard of Harnmaster?

Never heard of it.  I'm looking at the tables right now and this giant arcane wall of numbers is exactly the kind of thing I want to avoid.  That's why a lot of things are abstracted out.  I want to keep play smooth and I want the descriptions to provoke the imagination while staying true to the spirit of what I want to emulate

The ideas you're describing -

"sharply turning cuts, effortless sweeps, complex strike combinations, precise aiming, responsive parries, repeated jabs, ripostes, trapping and disarming an opponent's weapon, and tireless moulinets [...] A two-handed sword can singly fend off a group of attackers. A war sword (longsword) or pollaxe can dominate in dismounted armored combat. A buckler can protect a combatant while closing the gap. A bastard sword can be quickly drawn at the hip to assume a guard and used effectively against all types of weapons. A mace can bash helmets with simple techniques. Spears have a reach advantage and can be dangerous in untrained hands or tight groups. "

- do tend to lead to a giant arcane wall of numbers, though. It's simply that if you mention them as anything other than cosmetic description, you need mechanics for them. And for the choice to be meaningful, it has to have different numbers attached to it, with this being useful against that, and so on.

If you want such details, then you're eventually going to develop giant arcane walls of numbers. If you don't want the walls of numbers, then the details will be just cosmetic, and you'll go for abstractions. After you've written a game's worth of mechanics a few times you start to understand why Gygax just went for hit points In principle detailed realistic-themed play doesn't have to have excruciating detail, but it's really tricky to do it without it - if you rely just on the rules, anyway. If you rely on GM rulings then it's a lot easier.

Lots of people like giant arcane walls of numbers, Rolemaster, Harnmaster and so on were quite popular. Some prefer their numbers front-loaded in character generation, as in GURPS or HERO. It's an entirely valid way to game, but it may or may not be where you want to take this.

Let's look at this another way: what games have you played in the past, which ones did you admire and which ones didn't you like? Either here or in a notebook, bullet point or tabulate the things you liked about them and didn't like. If you share that list with us then perhaps we can advise you on a game system which already does this - you may be very widely-read in rpgs, but John Kim lists 1,101 of them, plus another 500 free ones, unless gaming has been your full-time job for decades you probably won't have played them all, but someone else might have. What's suggested may not be exactly what you want, but it may have ideas you can steal for yourself!

The question that influences my decisions is "what can be abstracted without rendering the system cosmetic?"

You quoted all those individual cuts, sweeps and thrusts and such I mentioned.  Well, *those* are abstracted because the guards and action/response attributes already model the combat results in a way as to be accurate to the source material: the manuscripts and modern reenactments.  Take a "master strike", for instance.  I don't really need to define the move any further than a strike that knocks the opponent's attack offline, do I?  Further, we look at a spear: it can perform repeated jabs.  What is the function of repeated jabs? To frustrate an enemy's guard, but the weapon itself is already modeled in such a way that it frustrates an opponent's guard and prevents them from attacking, so modeling the jabs individually would be redundant.  "What if I want to do a powerful thrust?"  Then there's a universal mechanic that allows you to sacrifice composure or fall out of your frame for actions in general in order to add to its effect.

We look at hit points from DnD.  I asked why a separate stamina mechanic was necessary, and came up with composure instead.  And I can keep that value manageable by being realistic: getting more and more health at level up makes it seem like a powerful character can somehow survive a perfect blow with a deadly weapon, so the GM never has to keep track of different maximum values for each npc because humans are still human regardless of competence.

Skillsets, do I really need to break down ability into war sword, two-handed sword, arming sword and the like?  Do I really need the player to choose from a dizzying tree of unique perks and passives?  Instead, I've listed different guards you can develop (these often apply across weapons in the manuscripts), in which some weapons are better suited to some guards then others.  Therefore, ability with individual weapons will naturally vary.