I'm new to the WoW-Heroes forum.......(prepares for the incoming n00b initiation ritual)
I think it'd be awesome to have a tab added called "World" which is identical to the "Realm" tab; where you can see where you rank in comparison to other players of your spec/class.
Understandably the database search time would be massive for something like this, but to make this doable, reduce bandwidth usage and reduce search times; try this:
Display top 100 ONLY for each class/spec. If a player isn't in the top 100, there should be a sub search field that they can type in their character name, realm and select region (US;EU...etc) apply appropriate class/spec filters in the existing filter setup, hit submit button and it will find and display your characters gear rank score compared to everyone else of that spec/class in the World.
That should minimize bandwidth usage for this feature greatly, enable players to see how they compare to their World Wide peers, and add a cool new feature that many I'm sure would enjoy.
If this has been suggested before; hopefully my suggestion was better thought out and is something you'll consider to be worthy of your time!