Teamchat ought to be fixed. Maybe leave an option for old style teamchat, but at least make it work properly by default. And put brackets around it for even players, like it should have (unless this can only be fixed clientside).

When thinking of other "bugs" to fix, remember that some things work more like unintended features. This includes things like bouncers going through walls, and trigger zones only affecting local players vs trigger crates affecting the entire server. This sort of stuff is strange, but has become somewhat vital to a lot of the gameplay. Plus if stuff like bouncers were fixed, weapon tragectories would be different between client and server.

Bascially all I'm saying is bugfixes are good, but be careful what's really being fixed.