Great campaign story; Immersive multiplayer; Excellent straightforward controls
Having played most shooters since Wolfenstein 3D, I can safely say that very little in the genre surprises me. At the point I played this game (which is more than a year ago), I was beginning to lose my interest. And then Modern Warfare comes out, and surprises me.
With an engaging storyline in the campaign, combined with a highly immersive multiplayer experience -- time literally flew by in the days I played this. I don't even mind how short the campaign was -- I finished the game once, then went back and did it again just for kicks.
For a change, a shooter really had a story to share and implemented it perfectly. The entire campaign puts you in different situations using different personas with missions so challenging that the end of each one always leaves you wishing there was more to do. It dragged you into the story, and made you empathize with the characters enough to believe that you too, in fact was just as vulnerable as they were. Needless to say, this all comes with terrific production values: The graphics were great; the sounds were fantastic; the dialogue very immersive.
For me though, it was the multiplayer that made this game shine the most. The reward system is designed to make you feel compensated in the short term (each round), the medium term (the entire night), and the long term (running stats). The internal challenge to accummulate kills without dying gives you something to aim for within each round, and the bonuses awarded make you feel rewarded well. You have very interesting MAPS to play with, and all of them require a unique strategy that play out differently depending on what kind of weapon you're holding. All of these small touches are, well, minor... but they all combine to form a terrific multiplayer experience that makes you want to come back for more each night. That, to me, is the barometer if a great game: You don't want to stop playing it.
In fact the only reason I stopped playing this was when Modern Warfare 2 came out --- but that's a different story.