TF2. See the Demopan thing for example. It's become more about hats and trading than actually playing for a lot of people, and whenever there are promotion items for another game, they all pay for a new hat and get a free game.
For example, the Killing Floor christmas event. If you had both games over the holidays, you'd get a pyro skin in KF and some pyro items in TF2. That brought a lot of players over, sure, but they made the game nearly unplayable. They'd go into harder games without any leveled perks (whose bonuses you need to take on the tougher enemies), buy the weapons that were practically useless for their class, etc. That can be blamed on being new, but these guys absolutely refused to listen to any advice, and kept right on hurting their teams.
The last few updates have only fed this. See: Mannconomy, Medieval update, 90% of the community hats, etc.
Edit: for clarification, I'm referring to the general playerbase portion of the community. Most of the modding community is all right, and it's really Valve's fault for not playtesting the community weapons and not bothering to restrict community hats to fit within the game's context, so I don't really blame the people who make them as much.