My best and worst are tied together; IE I'm damn at improvising, every single one of my adventures ends up being run in a completely different way that the one I had in mind, this is in part because I come up with a lot of things on the fly that ends up on making the session really long, my sessions are always at least 6 hours long, and that's my worst, the fact that the plot always gets delayed due to sidequests, etc.
I'm also quite good at creating tension when the players least expect it.
And I need to improve my NPC's, I tend to make them a lot like generic computer npc's that individuals with different personalities, except for the villains, those I work them a little harder.

Best:  Foreshadowing.  More than once, I've had players near the end of a campaign say, "Why didn't we see it?  It was in front of us all the time!"

Worst:  Descriptions & details.  Usually I'm okay, but when I get flustered or fatigued, I'll invariably totally bork up some important detail or plot point, and will then have to backtrack to keep everything from falling apart.

Good: People like playing in my games. I have no idea why. I guess it's 'cause I keep things moving. I run D20 combat for 12-16 with about as many antagonists in a half-hour or less. From all the bitching about slow combat, looking up rules (I use non-core... almost to the exclusion of core), and big groups; I am left with the impression that not many share this talent.

Other Good: I draw things. I don't know why, but my players like it. I'm also quite obsessive about homebrew rules and settings. My DnD plays a little closer to Star Wars D20.

Bad: I can't describe for shit (thank God for being able to get by on drawings). Also, I can't really get a campaign to last longer than two months (mostly poor planning... getting 12-16 in the same place at the same time is a bit rough).

Good: Player freedom & empowerment -- no "Plot Hammer". Really enjoying & encouraging the awesome stuff players do. Playing NPCs (even though I think, frankly, most are fairly stereotypical, the players engage & remember them), pacing and constant pushing towards conflict.

Bad: I'm bad (but trying to improve) at making notes during a game. My constant push towards conflict can sometimes leave games feeling a bit "breathless". I get bored playing in the same system/setting for more than 5-6 weeks, so I simply can't run good campaigns which last any longer than that.

Ok so now.. you know this forum?

This forum should be about how to figure out ways to fix all of the "bad" parts, and help you do the "good" parts even better.

RPGPundit

I'm very good at NPCs, improv, and random, funny effects. And detail.

I'm very bad at organization.
I'm also bad at running unevenly. I try to be relaxed about the rules and make it easy to play, and the Rules Lawyer (Perfect Brian) and the munchkin (Ace) take advantage of me being lax and try to destroy the game and world. I change and try to run very tightly and rules bound to keep Brian & Ace in check, and offend or turn off Phindar, the other good players, and all the newbies. I finally resolved the Brian dilemma- he left the table, but it's very hard to only have to apply the rules to those who need rules in order to behave properly at the gaming table, and not apply the rules to those who are already mature enough to know better. sigh.
90% of my House rules were written for exactly 3 players.