Me and a friend of mine connected our computers together yesterday and did our first multiplayer flight ever in FS2004. We noticed some issues.
1) when we created the flight, we chose the same airport and the same gate: both planes were put in the exact same spot, so one of us had to activate the slew mode and move the plane away.
2) at the gate, on my display, my friend's plane looked suspended above the ground level. On his display, my plane looked buried in the ground (the wheels were not visible).
3) ATC was disabled, and there was no AI traffic.