1) Multiple desktops. The main reason for, say, having multiple firefox windows open is to separate tasks, like work and play. Separate tasks are usually done in different desktops, in linux. In win 7, you open a new window of the application, if it has tabs at least, to separate tasks. It is a minor nuisance to open the new window, and a new desktop would serves the purpose much better.
2)The start menu. Besides the fact that is doesn't have an add programs thing like linux (repositories FTW!) it would be great if it used categories similar to linux for apps, so you wouldn't need to search for every app. I mean, the most opened programs is silly, as presumably you've added them to taskbar, and there is no longer a need for programs you want kept there to be at the top part. Oh, and the search function in the start menu should be like gnome-do; with many, many, many functions and opportunity for addons.
But really, I think win 7 blows the competition out of the water ATM. Window peaking, searching at the start menu, uber transparency and movement effects, many functions in the start menu, and the nicest window borders I've ever seen.
What's not to like? (About the UI, I mean - still plenty of reasons to ditch it for linux besides)

