Every option deactivateable.
Enhanced Serverbrowser with Serverpreview and freely configurateable favorite server section. (+ Searchbar, Clan-search, free slot-wait, ..)
New HUD, new loading screen, improved scoreboard..
New playermodel skins (teammates and enemies) which are ingame changeable. (+ New Flags, other weapons..)
Freely configurable look'n'feel (Backgrounds, icons, cursor, ..), you can even put your own stuff inside.
Demobrowser: see the results on the fly, jump to a specific event (e.g. if someone said something, scored, ..)
Ingame IRC, renames screenshots and demos automatically, automatically takes screenshots (eg after duels or clanwars), ...
And of course open source.
Available for Windows, Mac and Linux.
have a nice day :)