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. 

**Edit**

I `+1`ed everyone. Thanks for the great answers!