I live on a college campus which blocks Xbox Live (I know, it sucks). As with Cod 4, there is no support for more than one player per system in a system link. This makes it pretty much useless. Only so many people have xboxes and only so many people who have systems have the game. You are basically stuck playing 4 players on one screen unless you want to LAN a 1-v-1.
Additionally, Spec-ops mode (which unfortunately steps in for the co-op mode and doesn't just add to the game) wont let you system link at ALL. This is absurd in my opinion.