1) The ability to play through the campaign co-op.
2) Kill streak rewards that aren't quite so good. Predator missile - balanced. AC-130 - not so much.
3) Spawn protection (nothing like spawning outside under a harrier over and over again)
4) More Spec Ops would be awesome
EDIT: Dedicated servers and a more polished multiplayer lobby. Take the example of Battle.net
2) Kill streak rewards that aren't quite so good. Predator missile - balanced. AC-130 - not so much.
3) Spawn protection (nothing like spawning outside under a harrier over and over again)
4) More Spec Ops would be awesome
EDIT: Dedicated servers and a more polished multiplayer lobby. Take the example of Battle.net