I can be a hard guy to DM for...
I have a bad tendency to bring my "A" game to the table, even when doing a beer-and-pretzels game. I sometimes forget to dial it down to the level of the current game and end up unintentionally hijacking it.
One of the major symptoms of that is I sometimes get grief from the "screw the plan, I'm charging in" guys (and even occasionally just the "hey, I've got a plan, too" guys) because I often end up being "the guy with the plan"...
And on the other end, it leads to an unfortunate tendency to kick the DM right in the plot hole, lol.
If they missed it (whatever "it" is, whether it's a rules technicality, a flaw in the bad guys' game plan, or the DM casually mentioning there was a piece of string tied to something unimportant) chances are I've spotted it and am currently considering six different ways to exploit the hell out of it with my +5 monkeywrench.
EDIT: And sometimes I get annoyed at myself for having to expend effort to make sure I'm not turning a game session into The Mad Jack Show.