If a website works in Firefox 4 and in Google Chrome 10, what could potentially cause that website not to work (broken layout or broken JavaScript) in IE9? What limitations and differences does IE9 have, aside from vendor-specific stylesheet rules?

Yes, that is a painfully vague question — that's because I am not asking this question from the perspective of someone with a specific problem! I'm asking this question from the perspective of someone with a working website who does not have access to IE9.