The only problem with Modern Warfare 2 is the multiplayer.
Infinity Ward have introduced IWNet which has replaced dedicated servers.
IW claims this is will combat cheats. The reality of it is that there is more cheating going on than when previous Call of Duty's had dedicated servers.
IWNet can select anyone to host a game, so if someone has a poor upload speed your game will lag big time.
Other problems are if someone is the host and they quit the game, the game will pause while looking for another host (migration).
You never had these connection problems with dedicated servers.