BioShock 2. If ever there was a game that begged not to have a sequel, it was BioShock. The story encapsulates perfectly, there is no need for anything more. While BS2 is not a bad game its very existence is an insult to the original.
Skyrim. The game is just too damn big. I don't mean the world itself, I mean the endless side quests, the 100+ dungeons with 50,000+ burial urns each containing 3 septims, and so forth. I am a compulsive looter who feels the need to collect everything. It's unlikely I will ever finish the main quest.
Left 4 Dead, either one. Rage quitters. I've lost all three teammates after we lost the very first round of a versus game by less than 100 points. Holy freaking god, I can understand quitting after you've been curb-stomped, but we did not do that badly. Is it really so difficult to imagine we might make up those points later -- or even lose the match, but still have a good time? Nope, gotta quit and find "a team that doesn't suck," i.e. a team that will carry your whiny ass on its back -- for all of five minutes, until you rage quit on them too.