Pros:
To start, I LOVE THIS GAME! before modern warfare 2 came along modern warfare was the best selling console game of all time. the campaign is so fun and you get to do everything, from being rambo, busting in doors blowing stuff up and just shooting everything to skulking in the shadows and killing 0-2 people in a single level. the multi-player for this game is surprisingly balanced. You get to use perks which are hard to explain but they help a certain part of your play, there are perks that increase bullet damage, increase sprinting duration and so on. In multi-player you get killstreak rewards based on how many kills you get in modern warfare you can't decide which streaks you want specifically like you can in modern warfare 2 but modern warfare, in my personal opinion has much better maps and a lot of the bugs that will leave you frustrated in modern warfare 2 don't exist for modern warfare. In modern warfare you can't have a shotgun in the off-hand like you can in modern warfare 2 which is another big factor that balances gameplay.
Cons:
The bad thing about this game as it is with all CoD games is the lack of dedicated servers. Every three or four games you will experience epic game lag that kicks you from the game either back to the lobby or the main menu.

But this game is well, well, well worth the occasional kick!