Now on to the downsides.
Their video tag will only play .h264 files. Originally the specifications stated that the video tag would use the open source OGG codec but then Apple complained and so any codec could be used. Not that the fact both Apple and Microsoft own parts of the .h264 licences or anything. Now Google has purchased a company that was working on a new codec called WebM and has made it freely available with Firefox already supporting it as well as Chrome. Microsoft say they refuse to support WebM until Google can guarantee there will be no patent claims against WebM in the future. This is typical MS hypocrisy since their own licence for letting you use .h264 clearly states they offer no such guarantee themselves.
No support for the text-shadow CSS despite the fact Internet Explorer has offered text shadow as one of it’s filters since IE5.5.
Flash can be very laggy. My current favourite game, Zuma Blitz, is damn near impossible to play due to it being so jerky. This is possibly down to the hardware acceleration as Firefox 4 also suffers from this.
Still I think for most people who only use IE this will be a major breath of fresh air.