It has native support for H264 video as well as MPEG2.  This is great news in Europe where H264 is used for HD (and some new SD) digital TV services. (Vista TV Pack had H264 support pulled between Beta and final release, and it was only possible to add it with a hacked version of a Beta-release DLL, and it wasn't that stable, and stuff like thumbnails didn't work well)

If you are comparing with Vista Media Center without TV Pack then it also adds quite a lot of extra digital TV functionality (particularly for those outside the US).  In the UK it now supports Digital Interactive Text, both DVB and World Systems Teletext subtitles (including recording them), DVB-S native tuner support (FTA-only and not DVB-S2 at first glance it appears) and H264 video (not sure if HE-AAC audio is supported - which is required for quite a few new DVB-T territories)

I installed Windows 7 Beta over the weekend, and BBC HD (H264 video, AC3 + MP2 audio) appeared on my first DVB-S scan.

Some of the UI concepts have changed - INFO now works differently.  Rather than scrolling up and down a list of options and then selecting them to change, you scroll left and right through the options and up and down to select, but once you've changed one INFO option you can continue to scroll left and right through others categories.  Quite different, but a bit more flexible.

It FEELS snappier and more robust after the first few hours.

One very nice feature is that the progress bar appears snappier, and you can scrub along it with your mouse - not sure you could do that before?