There's clearly a bunch of new stuff in Flash 10, I'm seeing a bunch in the Graphics class so far. Struggling a bit to reconcile it with what I know already but it looks useful - render lists and 3D transforms, etc - however I wondered if there are good arguments for targeting Flash 9 since I can definitely manage without all the new stuff I don't know. Like support on non-Windows devices, wide adoption of Flash 9 Vs 10, etc.
1% more people have flash player 9 and below, than have flash player 10. http://www.adobe.com/products/player_census/flashplayer/version_penetration.html
It's a shame that these figures aren't broken down further, but you can assume that by publishing for player 10, at most, 1% more people will have to upgrade than if you targeted player 9.
As well as new classes being available, player 10 also had a lot of speed improvements.
To answer your question, none that I can think of :)