Gone through periods of online gaming, but its all about story for me, game one in CS is the same as game 3million. A story is immersive, which is why MMO's to a degree are now what I play for the majority of my "online" gaming. With that said thats why Lotro wins out over others I've tried as there actually is a main storyline you play, right throughout the game, while WoW and things the general story is theres a war going on, but you don't really take part in that story till end game, which bores me to death.
RGP's single player for me compared to FPS type online gaming, a game now and then is fine but its boring, also usually painfully easy and same every game.