1. Search inside a text area or input tag.
2. Be fast
Using the Sun Spider text on the same system IE9 is about 10x slower than chrome 12.
3. Use HTML5
Some HTML5 like canvas and video tags are supported but IE9 does not support any html5 form validation or new input types or things like autofocus. We’ll still have to rely in scripted solutions and IE slow JS engine.
4. Use CSS3
IE9 does support some CSS3 but the cool things like gradients, transitions, animations, border-image, and text-shadow are not supported.
5. Use WebGL
Any games made with this new technology will have not be enjoyed by IE users.
6. Offline file storage
Web apps will be able to do cool things but not on IE.
7. Load many existing websites
For some reason lots of sites which worked fine in IE8 choke on IE9 if they’re not updated to support them. Anything using prototype JS has to be updated to the latest version which supports IE9. I never have to update my site to support the latest version of Chrome or Firefox why does IE9 have to break existing compatibility. ( I know there’s compatibility mode but a good browser would work on any site without requiring the user to click the broken piece of paper icon.)
8. Run on anything but Vista and Win 7
Firefox 4 and Chrome 12 both support Windows XP, Linux and OS X. I think Microsoft is too lazy to support anything more.