I watched my flatmate play MW3 for a couple of hours last night, and really it looks like little more than a mod. I guess the same could be said about most of the COD franchise, but really there are so many games in the series now there isn't much room to innovate. Xbox360 obviously being the limiting factor in the equation.
The main good things I noticed was that it actually shows your killstreak count and it's easier to compare weapon attributes in the menu. Also you dont have to use your killstreak to access a higher level one, you can toggle them instead which is nice. e.g. you used to have to use up your spy plane before your could call a chopper gunner.
Overall not it doesnt really jump out to me as a must have game, it's just COD in a new flavour. And some of the sniper rifles seem a bit too easy to use too.