The first thing you will notice on Vegas Pro 9, after installing or upgrading it, is the darker interface. The darker palette allows the human eye to have less contrast between the video and the interface, and therefore make more informed decisions when color correcting or color grading.
In terms of new format support, Vegas Pro 9 has a few interesting surprises for us: it can now capture XDCAM EX footage in MXF format (e.g. from the Sony EX1/EX3 cameras), while it now offers supports for RED's .r3d format! Sony implemented a pretty in-depth dialog/UI to control the RED RAW format as you add it to the timeline. Of course, in order to open these files, Vegas now had to support video resolutions up to 4096x4096.
On top of all that, there's even better support for AVCHD footage. DV, mpeg2 HDV and MJPEG also worked as admirably as in the past. The only problem we had with format testing was the slowness, and often crashiness, of the application when editing .mp4/.mov files from HD digirecorders, like the Flip, Kodak, Aiptek, and Sanyo Xacti series. This problem has been plaguing Vegas for years, and I was surprised to see nothing getting optimized in that regard -- at least for stability, if not for speed.
On a high note, the 32bit floating point processing feature is now better than ever. Now you can switch back and forth between the faster 8bit processing and the (slower, but better) 32bit processing with a flip of a switch without undermining the quality of your video. It is now suggested that editing can happen in 8bit, but export in 32bit at the end -- which is what matters.
And if you like these... big numbers, here's a good one. Among new image formats now supported by Vegas (e.g. OpenEXR, MS HD Photo, DPX), the app can now load images much larger than a gigapixel! This allows to zoom in to a picture, and crop, without introducing pixelation.
On the more user-visible features side, there are now new plugins and transitions added to the existing collection. Users can now use visual effects like the ones found on After Effects or FCP: glint, rays, defocusing etc. In fact, I smiled when I saw the "rays" plugin announcement, because just a few weeks before that, I was watching a music video (directed by online friend Mark Pontius, drummer of the band) using the same plugin via FCP, and I was thinking to myself "hey now, how could I compete with this? I can't!". But Sony had this surprise under its sleeve for me. Also, Vegas comes with a powerful SDK that allows third parties to develop their own plugins.
Vegas Pro 9 comes with DVD Architect 5, which supports full Blu-Ray authoring, in mpeg2, AVC, and at frame rates of 50i, 60i and 24p. The Vegas Pro application itself can also burn the current timeline in an "AVCHD disc", meaning that you can burn HD video on a plain DVD disc using a plain DVD writer. AVCHD discs are supported by some Blu-Ray players (but not all).
Finally, the 64bit version of Vegas Pro 9 has seen a refresh with some fixes, but the fundamental problems it had with version 8.1 remain: no access to [most] 32bit plugins and codecs. I personally run Vista 64bit on my beefed up video editing station, but I prefer to still use the 32bit of Vegas.
Vegas Pro is a very powerful app for the money, as I stated above, but there's still room for improvement. For example, with Vegas 9 there is no good intermediate codec shipping with it (older versions were licensing Cineform), so I wouldn't mind seeing some (optimized) Avid DNxHD support in the future. The current implementation for Avid's free codec doesn't preview faster than 7 fps, which is too slow to edit properly!
There is no proxy editing support, and one has to resort to scripts or complicated workflows to go around .mp4/.mov editing. Also, doing frame adjustments is a pain: retiming footage, removing/adding pulldown, or slowing down/speeding up footage more than 4x is a matter of long tutorials that the user has to hunt online and carefully follow.
Maybe my biggest gripe with Sony in general is the fact that they haven't released an update for their consumer version of Vegas, Vegas Platinum 9, to fix their 100% reproducible HD SonyAVC/Blu-Ray crashing during exports. Hopefully in the near future.
Overall though, both Vegas Pro and Platinum 9 remain the best solutions for people with a strict budget but who require immense power and flexibility during editing. Personally, I always suggest Vegas to people who ask me which editor to buy. Sure, Vegas has its problems, but most other PC editors in its price range have an order of magnitude bigger problems than Vegas, so the choice quickly becomes clear (especially if you are using standard camcorders and not digirecorders).