Depends. I prefer single player for most games, but if we are talking co-op, I think that can make some games better. Like RE5 mentioned before, though the addition of the AI character in single player sucked. If you play the game co-op it becomes one of the most awesome games this gen. I actually want more games that I can play through the story with local co-op, esp survival action games.
It can hurt some games by splitting resources to add multi player, but it didn't hurt Uncharted 3, or ME3 in my opinion. As for God of War, I can't say. I just started playing the first one. Not sure about team deathmatch, but I would not mind playing through the game in local co-op. Though the platforming parts would need a major revamp to make co-op work.
