A Custom Fields UI
I think really custom fields with custom UI is really a big requirements for a lot of people. I mean, compulsory, repeating, grouped, multiple images, etc.
There is a bunch of plugins trying to provide the functionality, but they are all add-ons and have to play catch up to new WP features every time. Magic Fields seems reasonably good as a start point.
And they would make perfect addition to the new Custom Posts.