No single-player: Yeah, games that are only multiplayer-based hold no interest for me; if there's no single-player mode then I won't be buying it. I don't do multiplayer in general, it's just not my thing. The only exceptions are local multiplayer modes, like in fighting games.
Similarly, games with monthly subscriptions or always-online requirements are also games I won't be buying, simply because I just don't care for them. Also, I don't like the idea of having to constantly pay for games - I like to buy a game and that's it, thank you. And as for always-online, I guess just sometimes I don't want to have to be signed in before playing.
Other than that, I guess just generally horror games. If a game is a horror then I'll pass thanks, not my kind of genre.