Are you playing music while surfing YouTube? Playing a Flash video mutes the music, and stopping the video unmutes it. Got an incoming VOIP call? Your video is muted. There are reasonable defaults, but the behavior is configurable.
This project is still young, but has great promise and does something immensely useful for desktop users.
Audacity audio editor being the default MP3 application? How silly would that be.
Of course, Apple has had Cover Flow (link to video) for a while as a way to easily get large previews of files (and even watch videos). Goobus is kind of a Cover Flow clone for Gnome. It's a little unwieldy because of the hotkeys necessary, but the developer is asking for someone to help develop a Nautilus plugin to integrate Goobus better. Want to help out?
Tracker SearchTracker was included in Ubuntu early on in its life and was panned as a resource hog. Ubuntu went so far as to disable Tracker search in Nautilus. Fortunately, the guys at Tracker weren't detered and have gone on to develop a n extremely fast and SPARQL-compliant indexing / search engine. The development branch even does thumbnailing, album art lookup, and multi-lingual word stemming.
Integrating Tracker search would allow a lot of application to offload independent work that they do now onto Tracker, making that information available to all applications equally. F-Spot could use it to store tags and meta-data (though why would you really need F-Spot if Tracker and Goobus were adapted to use with Nautilus?). Totem could use movie and music info plus tagging to create a nice little music manager, or Rhythmbox could query Tracker instead of keeping its own database. Even menus could use Tracker to speed up the menu time by querying instead of parsing files. Recent files would be available to all applications by service (Video, Music, etc.) or by MIME-type. Epiphany, Gnome's default browser, could move its tagged-based bookmarks into Tracker and keep browsing history there.
Wow, that's a lot of info. Gnome's Zeitgeist would have access to all of it in one spot. Zeitgeist would no longer need to add filters: the applications would handle that part.
Gnome could then move to a tag-based view in Nautilus. Tags would look like folders to the user, with the possibility to switch between tags and physical disk layout. Users would not be overly confused by a new desktop concept, but their desktop experience would be much more enjoyable.