At the very least, 4 channels: All, Server, Team, and Buddies.
All: If you feel like talking to everyone. This would be a general chat.
Server: Talk to the people on your server.
Team: Good for TDMs and such
Buddies: With RTB's new Buddy system, this would work perfectly.
And possibly a 5th channel, Direct chat, which would allow you to "Call" an ID for a 1-on-1 chat. Kinda like a phone.
Also, an ability to mute players by typing in their ID's.