Adobe have added a lot of new features to the player. I am just gonna highlight what stood out to me.
3D Support & Effects
There is now support for 2D surface in a 3D environment. This mean you can animate a 2D object in 3D space natively in flash with out the need for a 3d party engine. If you would like to use 3D object like a cube, sphere or teapot with out have to construct the basic objects your self a 3rd party engine is still needed. The effects in 3D are like the filters only applied in 3D and seem to have a more robust support which incorporates the hardware acceleration.
Graphics Card Support
Finally, hardware rendering that does not require full screen mode. It needs to be enabled in the html though. It also seems that adobe have limited support to cards with Open GL 2.0 with GLSL capabilities. You will get a green square in the top left corner if there is any acceleration.
Text Engine Rebuild
Support for right to left and top to bottom languages. Maybe the engine will render a bit more like adobe photoshop which will mean matching flash to photoshop design will become a bit easier.
Vector Data Type
Basicly a typed Array. Should run a bit faster and take less memory.
Relevant links:
Release notes :
http://labs.adobe.com/technologies/flashplayer10/releasenotes.html
Install instructions(found in the release notes) : http://labs.adobe.com/technologies/flashplayer10/releasenotes.html#install