So many features we all have been waiting for years. Yes finally rounded corners :-P
Seriously: Massive improvement in JS performance, DOM events, XHTML, SVG, CSS3 selectors... we now need to test how all that works together in real pages.
Only for Vista and 7. Yes, they say that it's too early to talk about the final version, but if you don't want to talk about the supported platforms right now, how can we have any faith that you will respect most of your customers and won't leave them stuck with the previous generation like you did with Windows 2000?
Don't say that it's due to the drivers or anything like that. All the other browers work perfectly in Windows XP so that's a lame excuse.
You can test it by downloading the preview from http://ie.microsoft.com/testdrive/