Rubbish Hero wrote...
Co-OP is best done built specifically for it, such as Leftdead.
Resident Evil 5's problem was in attempting to merge them.
This can certainly be the case. However, I have quite a few FPS games that are easily playable in both modes with excellent integration. We'll get to see another good example of it next week. Actually, 3 good examples.
On June 22nd, Transformers: War for Cybertron gets released. I'm extremely excited for this game both because I've watched all the developer interviews (this game was devloped closely with Hasbro, not Michael Bay's crap versions) and played the demo (sorry, Xbox exclusive....lame I know). I think it's going to be the sleeper hit of the summer. Even the guys at Destructoid already love it. It's going to have single player with 3 player drop in / drop out co-op. Here's an example of where the single player will likely work just fine but having your buddies jump in changes very little and works just as well.
Bad Company 2 is getting DLC that adds a new co-op mode (similar to Horde in Gears 2).
Red Dead Redemption is getting DLC that adds 6 co-op missions.
I would honestly say (even though WFC isn't out yet) that all three of these games have EXCELLENT single player gameplay while also having the potential for extremely fun multiplayer experiences. I think it can always be done if done with care.