Here are a few suggestions:
- expand the shell capabilities (find, grep, easy batch processing etc.)
- include an installer/updater interface that allows installing commercial software (similar to iTunes Store; the precedent has been set; customer acceptance could be high if it wasn't Microsoft...) at a button press, including upgrade deals for installed software; that also allows updating all applications to newest version, not just Microsoft ones; that allows downloading/compiling newest FOSS software (Gnumeric, Abiword, Gimp, Gaim, XChat, etc.) at the press of a button (yes, include a compiler free of charge!)
- one single version priced at 99 USD to compete with Apple's OS pricing and eliminate customer confusion
Note that all of these points are about the user experience rather than the raw processing ability, whereas many Microsoft innovations due to arrive with Vista address the latter (e.g. WinFS). Also note my upcoming post about Apple.