GMs that callously make use of an iron-fist.

There seems to be this infectious belief on this site where a GM is the end-all and be-all to the smooth and contentious function and duration of a roleplay. To supporters and practitioners of this notion, it's assumed that a GM is inherently entitled to be unyielding in their judgement, and that they are infallible in their reason and logic. It's pretty ridiculous, and it's downright scary how much flak you'll take for just calling out the GM's bullshit for what it really is (especially if the other players start a lynch mob in the thread when you decide to open your mouth).

This does not mean be a willing doormat to every wish and whim of your accumulated player-base, but this also doesn't mean become this pompous asswipe that's practically begging to garner a less-than-stellar reputation across whatever roleplaying site you make anchor at because your word is 'law' (which is an indirect way of stating that "Even when I've been proven wrong, I'm right by default because I wield the most power in the thread.")

GMs like these seem to be badly confused as to what their title truly entails, for their station is really only guaranteed true legitimacy so long as they have players that are willing to stick around for the ride. 

Typically, no one's going to persist in a thread when there's a shitlord on the loose in it, and I don't think a GM is really much of a GM when his or her thread has nothing to show for itself other than a cool breeze and some chirping crickets.

EDIT: On the topic of bad GMs, I hate the ones that are addicted to making thread after thread---and then suddenly leaving these thread (without warning) right before the thread is about to get off the ground, or when the thread is only a few posts into its IC.