In addition to blocking modifications, MW2 ends the concept of dedicated servers. You know, where you host and friends can join your server, and you don't have to deal with random assholes.
Instead, you get to do matchmacking through Infinity Ward, like in xbox live.
They've also expressed an interest in charging monthly fees.
That coward John Carmack said he's glad that Activision took the heat, and that future id games will not feature dedicated servers either.
On the other hand, The publishers of the upcoming Aliens Vs. Predator game have come out and stated that their game WILL feature dedicated servers.