My failings are probably legion, but here are the ones that leap to mind :
Ignoring/Forgetting My Preparation: I'll have prepared a detailed history and character study for (as an example) the local priest, that gives his name, the secrets and plot hooks he knows, etc. And invariably, when the PCs meet him it's "Hi! I'm Erik the Generic Cleric!", and it goes downhill from there.
The possible fix : rereading my own notes before running the adventure, which I frequently skip.
Extreme Tactics: And by that I mean that the PCs opponent's are either pushovers (tending to follow the scream-and-leap strategy) or overkill (the impregnable fortress - I mean, impromptu palisade). And if I carefully plan appropriate tactics and strategies, I get bit by the first. (ARGH!)
The possible fix: Improvise as tactically as I do when I'm *playing*.
Impromptu unnecessary house rules: Like my 2e campaign where I ruled that 2 Continual Lights couldn't be brought within 1 inch of one another, or the spells would "short circuit" and explode.
The possible fix: Limiting myself to *necessary* house rules, and vetting them here before I use them.
Constantly Evolving Rule Base, or Neat-toy-ism: When the new supplement comes out, I go buy it, and invariably use a majority of the rules. This can lead to surprises for the players ("How'd he do that!?" "Well, he's a {blah} from {blah blah}") and myself (after giving PC new item : "Oh, frack. It can do THAT?"). Plus, rules I've never been satisfied with get tweaked mid-game.
The possible fix: Waiting until all the rules I *need* are out, and quitting cold turkey. These boards should also help provide an outlet for my inventive side.
Inability to tune adventures/plot for character *hooks*: Somehow, I just can't write an adventure to use a character's hooks - I can write to their abilities, but all that in-depth character background (that I love) tends to sit unused.
The possible fix: I don't know, maybe pregenerated characters?
Inability to write "epics": I just can't do it. I barely get started when I run out of ideas on how to get from X to Y, much less the climatic battle at the end.
The possible fix: I have no flipping clue. Any ideas?
Counter-acting these are the few strengths I have :
Superior knowledge of the rules: Well, at least compared to my players, I don't claim to be Caliban. I miss the old 2e ADnD Trivia game.
Evocative scenery description: I've managed to freak players and characters out just by describing wood that burns purple.
Well-constructed adventures and world: Things tend to make sense, interact properly, etc.