I would like to see a better, more robust use of Spaces. Perhaps setting different desktop icons for each Space.
Font mgmt is always an issue, and I wouldn't mind finally having a fool-proof way to manage fonts without the need for third party apps.
I don't suspect they'll bring back Apple Talk, but support for older printers via networking would be nice.
Slim down the OS, especially by leaving out unused languages from the start.
Add features like TextExpander and ClipMenu to the OS. I also would like to see Default Folder X functionality. (At the risk of displacing those small software companies who make these apps)
Can't think of too many "big" things, but there's always room for stability, compatibility, and other "little" things.
And yes, we should be able to run iOS apps on the desktop. If nothing else, it's a new market for $.99 games