So I've had this idea for awhile now, which was admittedly fueled a bit by Blizzard's attempts to make specializations more definitive (see the talent tree changes) and this sentence by Ghostcrawler:
"We view Arms as a soldier, like a career military infantryman. Think Gladiator. We view Fury as the wild berserker from up in the hills. Think Braveheart or Conan. It's easy to argue that bleeds or mobility fit one or the other depending on your point of view. Arms feels tactical (in the sense of having an answer for various situations) and Fury feels reckless. The Arms warrior has pet names for all his weapons, while the Fury warrior shows up for battle drunk and half clothed." (by the way, how do you insert a blue quote? ^^)
So Fury warriors are supposed to be barbarians that "show up to fights half naked", Protection warriors are stalwart defenders armored to the teeth and Arms warriors are somewhere in between, balancing protection and mobility. It would only be logical to reflect this on the armor they wear.
So here is my proposition: make armor change according to your spec. This may sound like a lot of extra work for Blizzard, but it actually isn't since my plan would be to have a preset model for each spec (sans the shoulders and helms of course, since they're the only two items that always have unique models) sharing the same texture. Something like this:
Fury
Protection
(notice the thicker armor and larger shoulder plates. The gorget-thingy would be part of the chest piece. Also, sorry about the sloppy PS work, I'm too lazy to spend time on something that's not going to earn me money )
Arms would keep the current models. Keep in mind that these are just ideas that could and should be expanded.
Similar changes could be implemented for other classes, such as Druids, where Feral druids would have a more beastly appearance, Balance druids would, I dunno, have feathers or moon-related objects or something, and Restoration druids would have a more nature-oriented set (with branches sticking out?).
Another idea would be to add spec-defined doodads to armor, like blood splatters for Fury warriors, flowers sprouting for Resto druids, lightning bolts for Elemental shaman, holy auras for holy priests and paladins etc.