Personally, I'm less concerned with any single feature from the Webapps/HTML5 spec, or a CSS3 module being implemented. I think others have given lots of great suggestions (*cough* canvas *cough*) that the IE team will eventually (hopefully?) implement in the coming versions.

What I would most like to see is a more open and ongoing bug tracking system, akin to what the Gecko and Webkit projects and Opera have. It seems to me that the IE team is only interested in feedback in very short periods of time, just before a release. For example, I found a bug in IE8's querySelectorAll implementation a few months back and had a hell of a time trying to report it. I eventually gave up. Here's some spammy forum that scraped the mailing list conversation: http://www.vistaheads.com/forums/microsoft-public-internetexplorer-general/471117-bug-ie8s-queryselectorall-implementation.html

Furthermore, I'd like to see the IE team pay closer attention to the specs. For example, just yesterday (May 13th 2010), the IE Team blogged about their CSS3 Selector Module support--which is awesome--including the ::selection pseudo-selector. However, ::selection has been temporarily removed from the spec. They're even showing a screenshot of text that is not in the current spec. (Navigate to http://www.w3.org/TR/css3-selectors/, section 7.3 and note that the section is intentionally blank.) It'd be nice to get things right the first time, or release experimental implementations with the -ms- prefix, rather than push out an implementation that very well might change.

(now that feedback is back "on" for IE, i'll try once more to report the QSA bug again, as the IE9 previews indeed fail my test page: http://miketaylr.com/test/ie8qsa.html)