CoD keeps the same elements. Point and shoot. There's obviously some small mechanics that change, but for the most part its just the same hand with a gun you see. While every Ratchet looks and plays different, but still has the same Insomniac signuature.
If you show me some screens of different HD CoD, I wouldn't be able to see a difference. The graphics don't change much either. I have played quite a few of them at my cousin's place, but I though we we're playing BO2, but actually we were playing MW3.
Can't really blame CoD for almost never changing. It makes money anyway because its a new game. Friends always buy the new game and you need that same game to be able to play with yer friends. Money, money, money is what aBBa (?) would say.
The only reason I buy a new COD game when it comes out is because everybody start's plying online on the new ones.