Any player who purposely leeches (your cat is destroying your room in the background is fair enough reason to have to step away for a second lol, I mean people who enter with that purpose), and anyone who demands and requires others to carry their weight. Those two overlap a bit but sometimes you have someone who's like "yeah I don't use tank stance" and they have awful gear and cannot hold aggro against a single spell and you ask for tank stance and they're like "nah just do less damage" (or insert whatever problem here, I'm not picking on tanks). Bad gear happens sometimes on accident and people can adjust a bit usually to compensate- I'm talking about something like a healer refusing to leave cleric stance back when it nerfed healing and had bad heals due to bad gear in the first place.. . . . . . Ima cry.
If someone doesn't play very well but doesn't expect others to carry them (trying to play, even if it's not great) then we're going to have a fun time struggling together, but if you're like "I died because the healer didn't heal me enough, they sux vote kick them" meanwhile they refused to dodge any aoes or didn't use any abilities beyond "1". . . yeeeh..... lol.
Now the standards go up a bit for savage/ex content but imo that's different. Bad to me is just someone who doesn't care about playing in any sense and expects others to play for them, often with a bad attitude.
Don't need to be a skilled player to be a "good" player in my mind. Nice + trying to play = I'm happy, let's play together!