Total lack of IE support is a bit...problematic.
IE9 is pretty solid in many ways. Runs javascript faster than Firefox. Jquery animated interface effects and so on run noticeably smoother and quicker in IE9. My main browser is Firefox and I like it, but I've noticed IE9's faster performance when testing.
Niche / hardcore game developers can potentially ignore this but if you are going for mass market you really can't.
As a web developer at a creative agency, it's impossible to pitch an interactive 3D simulation or data visualization project to a client if it won't work in IE. I'd be happy if even CSS 3D transforms were widely supported.