WHERE'S MPEG-2!!!
At the age of QT4, Apple promissed MPEG-2 decoding with Quicktime 5. Yet when it was released there was absolutely no MPEG-2 decoding through Quicktime, absolutely no word from Apple in respect, and most articles in the knowledge base featuring that promise vanished. Quite lame for Apple, trying to put that in a hole and cover it with some dirt and hope no one remembered that.
BUT, hey, after a while Quicktime 6 was on the works and it really would support MPEG-2 decoding this time (along with MPEG-1 -how smart-, and MPEG-4), really. Oops, preview release is out and no MPEG-2 decoding from Quicktime and no word about it in QT6PR's web page.
Crappy Apple...
As for DivX playback, I read that one of QT6 betas came with ffmpeg.component instead of what is included with QT6 which is QuickTimeMPEG4.component, and that with it you could playback DivX video 4 or 5, but NOT 3 (which is what the majority of films aout there are encoded with), as long as no other conflicting decoder, such as 3ivx D4 PR1 Decoder OSX, was installed. Still, the bug handling MP3 inside avis persisted in that beta (still is?) so you still needed DivX doctor to watch them properly.
You might want to give a try to MPlayerOSX, a port of a linux player which will let you play avis with DivX & MP3 without any need of doctoring or conversion. However it is somewhat buggy and will not let you, for example, playing non-fullscreen. Search for it at
http://www.versiontracker.com/macosx/