Ok, Multiplayer. Team Fortress 2, Counter Strike, Call of Duty (+ United Offensive), Left4Dead. All multiplayer games I have enjoyed more than I did CoD4.
Personally TF2 never appealed to me, CSS I tried many times and never really found it that great, Left4dead bored me after a few hours play.
COD4 MP has had me hooked since release, it's not perfect by any means, I could write a list of complaints about the game but imo it's the best MP FPS that's come out in the last few years.
MW2 was the game I have been waiting for, I would have upgraded my graphics to ensure I could run it as smooth as possible, now I may as well just buy a PS3.