for those of you having the

"Error user interface version 6 expected version 7"

problem, you might want to check your Gamedata/Base directory for outdated *.pk3 files. personally, i had a backup of assets0.pk3 file in my directory. i for whatever reason neglected to change it from the pk3 extension, and it contained dated objects that jk2 couldn't load. when i removed this file, it booted right up.

hope this helps.