As a player, I hate playing with people who make up the EXACT SAME CHARACTER regardless of genre. My old group GM loved GURPS and so I tried to take advantage of the numerous ads/disads/skills/powers/what have you while one of the other players was "a beautiful young woman of 25, who is good with technology and weapons and has eidetic memory." Every time.
On a related note: having eidetic memory and not writing things down, so you rely on the DM/other players to remember whatever it is that you're supposed to remember.
Players who stay in character ALL THE TIME: Generally breaking character and chatting during a break or something else is going on is accepted. This guy had a character who was hard of hearing, so HE played hard of hearing, even during breaks.
As a DM, I dislike: people in a group who are in a roleplaying situation and are getting impatient because there's no combat. Or who roll their dice and say "i got this number, so do I pass my Diplomacy check?"
Players who get so drunk that they are no longer contributing to the roleplaying/planning, but refuse to shut up. Srsly.
Players who agree to the timing/placement of games and then change things right after the game starts. For example: the host of our game reached out to some friends for a game I wanted to DM (Pathfinder). We got a group email going and we all agreed on 4 pm at the host's house. After a couple of weeks, one player emails me and says, "Okay, I won't be able to make it until 5:30 because of (personal issue). So, once I get there we can get started."
No, WE are starting at 4, because that's what we all agreed to and the game only goes until between 9 and 10. If you have something to do that legitimately important, than take care of that, because it's more important than gaming. I have since received another email from him saying he will no longer be able to attend due to custody timing of his children, unless we can change the day/time of the game, which took a lot of effort to figure out in the first place, so no, we aren't switching it, either.
Now, I may sound like a dick about this, but he was aware of this issue when he agreed to the 4 pm start time, so it's not like something changed.