Assuming you're still choosing to support IE7, that will be your new baseline.
This IE team blog post describes the improvements IE7 made over IE6:
http://blogs.msdn.com/b/ie/archive/2006/08/22/712830.aspx?wa=wsignin1.0
It mostly boils down to way less bugs to worry about (many CSS bugs, PNG transparency issues).
The new CSS functionality is limited but useful:
- background-attachment: fixed
- :hover on all elements
- fixed positioning
- min/max width/height
- selector improvements (child and attribute)