I'm not sure these count as habits, but I hate when people do them.
First off, I have an issue with people who don't re roll cocked dice. Especially if the dice rolled in an odd way (one side would have passed, the other side would have failed). Instead of immediately picking up the dice and rolling it, they don't. And there's usually an awkward moment afterwards.
Second, I hate when people go off into tangents talking about their army. No, I don't wanna know why you equipped your third tactical squad with a rocket and put it in a drop pod. I'm guilty of this at times, but I'm smart enough to know when the other guys not interested. I've seriously played games that have lasted two hours longer than they should have because of this.
I'm not a huge fan of the older veterans. This is fifth edition, and yes, I DO get a cover save for standing in a ruin, even if you can see the whole model. No, its NOT OK to houserule that if you're running a tournament. I hate older vets even worse when they try to call me out on my Codex, even though they don't play space marines. They haven't taken the time to read the updated rules, they don't know wtf is in your codex. I've had to explain no less than four times now that, yes, a tactical squad can regroup and fire its heavy weapons.
A little less related, but I also can't stand people who don't shower. Like, who don't shower and smell really bad. I understand that this hobby attracts people in the beauty department that are less than steller, but that's no excuse for lack of personal hygiene. I took the time to shower my ass this morning so I could come out and play; I don't expect you to glamour up for like a party, but I sure as hell don't want your grubby ass fingers all over my models. Go play by yourself.