Maps will be compatible. So will skins. Models will probably need new animations, and most mutators/mods will need to be updated slightly to work right.
Yea, a lot of things are going to need an update
Netcode changed
Core Code changed
I think that Static mesh handling changed a bit, but they said maps would only need to be opened in 2k4's UEd and re-saved to fix any potential issues
(Melaneus) Dammit, don't you know Furrysound's actually this hot anthro cat goddess? Who does server administration?