Making IE 9 happy
Microsoft does not support IE9 anymore, and neither do we.
If you're using jQuery, you need to update to 1.5.1 to get Internet Explorer 9 to work.
Apart from that there seem to be surprisingly few problems. Many CSS3 attributes work and no major layout problems at first glance.
I only recommend to take a look at your box shadows, since IE does render those a bit differently (generally lighter and offset by what looks to be half a pixel).