First of all, fix the chat. That seems to be the main issue at the moment. Razzorman pointed out the issues well above.
Second, there are other graphical issues too. When I die, where my name was is replaced by a white bar for the rest of the level.
Third, I'd be interested to know how the server works. I assume it uses one central server?
And in the future, how about making it so that, for instance, if one player presses a switch, the door that the switch opens is opened for all players? That would be awesome for co-operative levels. It might also break existing levels, so a checkbox option for this would be great.
Just my two cents
Also! About the chat, against dark grey it's invisible, so perhaps a background color? Also, it's covered up by objects, so if for instance there's water at the bottom of the screen, you can't see the last few messages.