Originally posted by another user
There are a few small conditions, most of the time that are beyond our control and cannot be changed.
Firstly, some games are region incompatible. This means someone with the PAL version of a certain game may not be able to play someone with an NTSC copy of the same game.
Also, games that are multi-platform (available on more than one console and / or PC) will never be able to play against each other. A typical example of this is Midnight Club 2, available on both the XBox and PlayStation 2.