Was just pondering in the shower at the weekend (as you do) about what still makes, say, MacOS X feel like a more cohesive desktop experience than even the latest and greatest GNOME in Indiana/Nevada.
One thing that came to mind was its integrated management of your media-- in pretty much any Mac app where you might want to insert or edit multimedia content, you can immediately access your entire music, photo or video library in a familiar-looking window and drag it over from there. It's built into the file selection dialog, too.
Of course, Apple only really let you manage your media library with their own software: iTunes, iPhoto, Aperture, iMovie, Final Cut etc. But it did get me wondering if there was a place for a freedesktop 'media library' spec, that would offer our users the same sort of quick, searchable access to their media content (be it local, remote, stored on Flickr, split across three DVDs, or any combination of the above) in any application that required it. And, of course, to do what Apple doesn't, and allow any app to manage that content, if it needs to do so :)