One of the things I love about any game is the story. If it's good and works with the feel of the game we're on a roll, and if it's all oer the place I'm probablly not going to care.
Luckily MW2 landed close to the middle. Without giving a spoiler alert, the game picks up 5 years after the end of MW1. From the jump everything still feels the same, no major control changes, and getting your shooting game back (especially if you've been playing any other shooters), takes a half second. By the time you've run the starter course a few times you're back to prime and ready to jump into the choatic world of cover and run.
They got rid of the infinite spawn system, and enemies actually think and respond to their situation. They thought in the first one, but if you didn't run to the next cover you would get pinned down forever, so it's nice to be able to kill, clear then progress. There are some sections from MW1 where that's almost impossible.
As far as weapons go, there wasn't a drastic increase in what you had available, which is good. They added some dual gun weapons, but they don't overpower a solid well aimed rifle. Everything is balanced with its pro's and cons (including accuracy and clip size). It's also nice to see some thermal weapons (though be careful you don't get an indicator of friend/foe), as well as the predator drones which makes defending a position real fun.
The story is a bit muddled, with motivation for some of the characters actions going unexplained. You'll run through and probably end up going through a second time because it was fun or you want the harder level achievement point, and that's when you'll realize the biggest con. The game is short. Maybe 8 hours on regular, and alot of that can be bypassed. It does seem that they added some "controversy" to enhance the story, but it's really nothing that couldn't have been done in a cutscene. Ironically you can also walk through that part without firing a shot, except when the cops get there, but you can also skip the whole thing without affecting the rest of the game.
IMO if there is a piece of a game you can skip without affecting the rest of the game, then you don't need that piece. I'm sure alot of people feel that way, but overall it wasn't even as big a deal as it seemed.
Ultimately, you will end up having fun, and ducking/fragging Russians from cover. If you haven't played Call of Duty 4 though, I would recommend it, as while this feels like more of the same, it also feels like its expanding on your existing COD skills. Halo 3 tactics will not apply!