Having a good multiplayer and/or single player is key to any video game to be success full. For single player it should be simple to learn the controllers and should not be the hard or easy to play. The most interesting part in most games are the cut scene or the button mashing scenes. Some of the block buster game that thrive on single player have low completion rates, the cause for this may be that people lost interest or they don't have time to play thought the campaigns.
The multiplayer is what brings people back to the game because in a way it is a new game every time you join one because you are playing against real people that will not act like the normal AI or bots.
It is interesting that people can make a living off of YouTube and muiltiplayer. They do this by uploading video tricks and tips or just by commentators over the video.