In this blog post I'll talk about what I think makes a great SP or MP game.
Firstly, SP games. Personally for me, they have to have a long storyline, upwards of 20 hours, and great controls. The controls don't have to be simple, as you can take your time usually in single-players. The game should also run at a lag-free 60fps (trust me, the old FIFA's on PS2 slowed down so much it was ridiculous) and have good graphics.
For MP games, they should still have a single-player game, which I would expect to be around 5 hours minimum with lots of replay value and 5 difficulty levels. The online should be lag-free, even if that comes at the expense of better graphics. There should also be a compact HUD, moved to the corners of the screen, and pack in as much info as possible.
I personally prefer MP focused games, but I will get a SP game if it has a unique element or a long storyline for instance Assassin's Creed II.
I'm also looking at some DLC. I've got my eyes ATM on the CoD 4 Variety Map Pack, which I will follow with the first Assassin's Creed II DLC (Sequence 12) then the Forza Motorsport 3 January Car Pack and finally the ACII DLC (Sequence 13).