I noticed a comment in another thead about new players being a problem on-line.
Most servers have stats active and it forces all players to have a stats account in order to connect.
These two things could be combined for varying degrees of control:
1. There could be rated servers - where you could control access by how "good" the player was - some kind of skill banding (yes we know stats don't really measure "goodness" in non-DM games)
2. You could use that to put a polite warning about the differences between online and offline games for players who have not passed a certain threshold test in the single player game - for example, something to the extent of: expect to be significantly out of your depth to start off with, watch the other players, find useful tips at url:..., etc
Something along these lines could make a significant difference to people's enjoyment online - dependant on limited ability to change stats accounts - tying stats accounts to a key or some such:
Rated servers would mean the opportunity to play games without new players grabbing a key team resource without a clue what to do with it (think ONS with Player and Player in the raptor and manta making a tour of the map looking for bunnies).
Rated servers would also mean the opportunity for learners to play other learners without significantly better players ripping them to shreds and putting them off the game before they have a chance to learn.
Of course free-for all servers should still be there!
It looks to me like there were plans for rated servers in UT2004 - that almost made it to the final game. There are hints in the code and the server join screen.