Come on, you have to admit that Black Ops did a lot of things that MW2 didn't. The new game modes are a nice touch, combat training is fun to mess around in, the graphics have a lot of colors now and are appealing on my tv screen instead of just dark colors like the last couple of games. Drop shotting was sort of fixed, but not really, nazi zombies is fun, but I wish they didn't make you rebuy the maps if you had already purchased them in WaW and the story was just brilliant. I love the game as a whole and I'll probably never be picking up MW2 again. But, Cod 4 is actually pretty fun still.
Also, money system, TONS of customization.
I don't know what else you could possibly want...they advanced the franchise as a whole a lot with this game. They did everything perfect in this game and it turned out well.