The single player is basically the storyline. When IW tried to implement co-op into it, they failed miserably (because apparently it broke the immersion). So with Special Ops mode, it takes the best parts of single player that do work for co-op (e.g. snowmobile races). And because the missions in Special Ops don't have to be related to the storyline, they can pretty much chuck what types of missions that want in there too.
It's just basically a whole bunch of missions that are for co-op, that's all.
I don't really care if MW2's SP is going to be as short as MW1's - I had no problem with it being short. If you want it to last longer, you could upper the difficulty, or play it again in e.g. arcade mode. And even though it was short, it was pretty damn satisfying, so when people want more, it's a good sign.