Just got the game. There are a number of small changes which I don't like. Like, it doesn't have the same level of detail or small touches that the original had. I really liked the original MP campaign. The writing was pretty good and the characterization was classic COD.
I really wish that portraits were animated when talking to you, that sort of thing, like in the original where you'd see them yelling into the radio while explosions went off in the background.
And the unlock system is irritating because it is a very long grind.
Overall the SP campaign is quite good. There are touches of FarCry and Portal and Half Life 2 in the game. One level is like Zelda in a way that is pretty awesome. Again, I just wish that the game didn't have boss fights at all.
I really wish this franchise were more popular. It is the most fun I've had in a shooter ever. Like, I want to see animated series, comics, novels, etc. I think it's better than Halo easily...