- Spell check - that's something people use a lot and rely on. Right now I type this comment and make some typos here and there. My browser highlights them instantly. In IE9 I have to double-check each comment before sending it.
- Bonus point for taking grammar check from MS Office. It has some pretty powerful checks for some languages, notably Russian - it's amazing how far it goes! (To people unfamiliar with the matter: imagine a spell checker that checks only top 1000 words - that's how other grammar check in OO.o, Google Docs, etc. feel compared to MS Office) Actually as I think about it I become really sure that I would switch to IE just because of that feature.
- Redesign the download manager - it should be in a separate tab - not window. Trust me - that's so much more convenient and consistent!
- Do something with the top of the window. Right now it's just empty and takes some space. Either let it collapse in fullscreen so the tabs touch the top of the screen like in Chrome and Opera or put the page title there like in previous versions of IE.