Well most of the things are written on Microsoft’s website. But here is what we will suggest you in terms of a coder’s perspective.
Coders can breath now with the new IE9 supporting rounded corners and edges of a design. Earlier what used to happen was that with css if we set up rounded corners or edges in a design IE won’t recognize it and hence it looked pale (straight) in IE even though its appeared nice in other browsers.
The other new thing about Internet Explorer 9 is its recognition of PNG and advanced PNG. Earlier in 6 and 7 versions IE didn’t recognize PNG separately and hence transparent images were shown as non transparent.

But in new IE it allows PNG 8 and PNG 24 to be recognized as separate transparent images. So go ahead and design sites with these 2 great additions.

Now here is what you need to do in order to get people go for IE9 and discard using their junk versions.

Add this code just after thestart of tag for a webpage
And download the js from this site: http://code.google.com/p/ie6-upgrade-warning/downloads/detail?name=ie6-upgrade-warning.zip&can=2&q=
Hope this helps you guys.