Home Screen Customization (More than just colors)
People have mentioned custom themes but they are usually just talking about the ability to change tile colors. This is more than that.
Also see my other idea: A tightly Integrated Ecosystem http://windowsphone.uservoice.com/forums/101801-feature-suggestions/suggestions/2498317-a-tightly-integrated-ecosystem
I think a good start on themes would be to take a look at this site: http://windowsphonehacker.com/articles/themes_for_windows_phone_first_release-12-12-11
Also they figured out a neat solution for folders here: