See I played CoD4 to death, seriously. Then I purchased MW2, and quickly got bored...this is simply because I got bored of the first modern warfare game as you do if you play something so much; and MW2 is pretty much exactly the same game. This 'new' game looks and seems to play pretty much exactly the same too, so for that reason I personally won't be buying it.
I'm not saying it will be a bad game, because it will most likely be good, but in the sense that CoD4 was good...it will just be...well...nothing new, innovative or special...it will happily take your money though, and then some more for the inevitable map packs.