My main bad GM habit is that when players deliberately avoid plot hooks, I fail to improv very well for any real length of time. I also get pretty pissed off when they do that. If I'm the GM, it's my job to keep everybody entertained and put things out there for characters to do. When players go out of their way to avoid them, I have to wonder why they play these games at all. So I tend to get surly and get more and more anvilicious with the plot hooks (and the occasional railroading, should I get ticked off enough) until either the players finally bite or I call off the game. Sometimes I get whining about "well, my character wouldn't want to do that". Sometimes that's a fair complaint, though that's a relatively rare one given the way I operate. Most times it's just people who are more concerned with their own little roleplay world than either 1.) helping other people have fun or 2.) the fact that it ~is~ just a game and not a life-or-death theatre production. I tend to get worked up just thinking about it.
My Star Wars GM has two really bad habits which I've brought up with him on several occasions. First is his tendency to allow NPCs to overshadow the PCs. In the last campaign, Galen Marek (aka, Starkiller from The Force Unleashed) kept showing up to do awesome things while we watched. In our current campaign, he brought back a Jedi of his from a long ago campaign, made him a Master, gave him an uber sparkly super special lightsaber (white blade with a black core, also from Force Unleashed), and he's taken over. What's made this last one so irritating is that the GM has decided to ramp up the difficulty to near-impossible levels because to do otherwise would make things too easy for the NPC Jedi Master. Last session had a monster that was quite impossible for us to beat without the NPCs help (it could kill us in 2 hits and had only the most minute chance of missing) and it left most of us with kind of a bad taste in our mouth. I've told the GM that we're ditching the NPC Jedi Master first opportunity. Out of an airlock, if need be.
This leads directly into the GM's other bad habit, balancing encounters. The GM tends to make encounters too difficult and then has to cheat almost every night to bring it back from the verge of a TPK. The monster that nearly killed us all in the previous session? His claws had a damage code of 4d6+30x2 (minimum damage: 68) and his bite had a damage code of 4d6+30x3 (minimum damage: 102). The toughest PC's hitpoints? 78, ensuring that no PC could ever survive 2 hits and no PC could survive a bite without a miracle. He gets very adversarial when it comes to combat. He also tends to make enemy NPCs who are specifically designed to kill us, which tends to make battles rather frustrating. I've advised him multiple times to start easier than he thinks need to be for a challenge and then ramp it up a little (with good justification, like the arrival of reinforcements) if it turns out to not be a big challenge. And if he can't justify a ramped up difficulty, sometimes he should just let it go and chalk one up for the PCs. This is Star Wars, after all and there's plenty of times where the heroes just mow through guys with nary a scratch.