It was originally decided that animated gifs and such weren't allowed as they caused terrible lag on the computers of some (mainly those with 56k or crappier computers). Since animated gifs are sortof an unneeded luxury, they have been removed for now. (Though I was personally against this - as long as people don't constantly clear their cache or change AVs, they should only have a loading issue with avatars the very first time they load...)
Animations in sigs were generally disallowed as well, though some people don't mind it so much anymore - however, you still need to mind the KB limit for your sig.