The subject just about says it all. I'm excited to see some increased support for the HD stuff out there but there were several long threads dedicated to improving (reverting) the behaviour of some of the audio aux routing. I'm hoping this has been implemented.
The literature only touts the mixer as "New" - everything else seems the same. However, take a look at the new video features and then go check out the video forum. The folks over there are acting like Sony has given them the holy grail in Vegas 8!! I've never seen anything close to that type of user response to any audio release and I've been around here since Vegas 1 and Acid 1. Pretty obvious where Sony's resources and energies are focused. Of course that's been obvious for a few years now - just not quite as dramatic as Vegas 8 confirms it.
On either Sony's product description off of the main page, or the Video Guys write up, probably authored by Sony, they describe a re-written audio stack that supports multi-thread processing. This would strongly suggest faster audio rendering and lower latency on recording with plug-ins. Overall, this sounds like a massive overhaul of Vegas.