Traps whose one and only purpose seems to be hurting PCs -- ESPECIALLY those placed at the ENTRANCE to the dungeon.

Mushrooms: Shriekers, Violet Fungi, Myconids, and especially weird ones just growing around that if you eat them might be poison, permanently raise your strength by 1d4, function as a one-time Contact Other Plane, or turn your skin permanently yellow, with green polka-dots.

Death by random encounters

Monsters of widely disparate power levels on random encounter tables (Tegel Manor, I am looking at you)

Locked doors you will never find the keys for

Sublevels

Yes, and sublevels.

Quote from: MelanYes, and sublevels.

That lead to other planes.

an armory, that of course will be searched in hopes of finding magical gear

An interesting backstory to the adventure itself. Like 'This is the storied killer module by E. Gary Gygax for the RPGA tournament at the very first Origins'.

Cursed scrolls that send you straight to hell.

Mules as party members.

Mule

Mules attack, save, and gain hit dice as fighting men. They can use no weapons but their kick attacks as normal to the rear. They may purchase barding, and enchanted mule-barding may sometimes be found in the depths.

In addition to progressing in attack, saves, etc. as a fighting man, mules may carry a great deal of weight. A first level mule may carry up to 2000 gold pieces of weight unencumbered, up to 3000 at 9" movement, and up to 6000 at 6" movement. These numbers increase by 1000, 2000, and 3000 gp respectively for each level of experience gained.

PC mules have the ability to speak and read, though these are not common to the species. They may not write.

Dwarf Mules

Dwarves may also advance as mules. They receive all the benefits that Dwarves receive as fighting-men, except that they cannot use the +3 magic war hammer. They are shorter and stockier than other mules but aside from that are treated the same.

Quote from: MelanSublevels

Yes, and sublevels.

Oh, man.  One time Doctor Phostarius, the necromantic bard, and Bryo Phyta, the world's tallest half-elf, got lost in the Dungeon of Doom's third level.  They rejoiced when they found a stairs up, but those stairs led to sublevel 2a, which had no surface egress.  Until that point I don't think the players even knew that my dungeon had any sub-levels.  Fortunately for them, the duo ended up meeting some friendly gnomes who were able to direct them back out.  Bryo wanted to continue to cooperate with the gnomes, but the Doctor hated the wee gits and later tracked them back to their lair.  He lobbed a few fireballs into the place and then sealed the entrance with stone shape.  Good times.

Quote from: CalithenaMule

Is this an excerpt from an upcoming Lulu publication?

It better be coming up soon!

Quote from: WerekoalaWhat, in your mind, are the elements that make up a classic 1st Edition style Dungeon Crawl?

I'm not sure how classic this is, but one of the best lessions I learned reading some old modules (as well as some Hackmaster updates) to run my D&D 3.5 game (loosely based on B2 KotB and the Slaver "A" series of modules) was to create traps that would catch a careless party but also be easy for the denizens to avoid, if the dungeon is occupied.  

If the denizens are lightweight, make floors that give way under a certain amount of weight.  If the denizens are short, have traps that slice away above them.  If the denizens can see in the dark, have traps that utilize that by being triggered by light or avoided by markings only visible with darkness vision.  Or make traps that only go off if you set them off and are harmless if you just leave the trigger alone.  For example, simply put a magical glyph trap behind a curtain or a lever on a wall in the up position.  The benefit of this approach is that you get traps that feel classic but also stand up to scrutiny if anyone is concerned about verisimilitude.