"Rogues should be...well, rouge-like. Simple gear that you can sneak around in, nothing too glowy as with some of the past sets."

This. I'm combat, so I go for more of a martial look whereas assassination and subtlety would be more ninja-like, but the concept is the same: simple.

Since the Defias set has some holes, I've gone away from that gear for transmogging, but I still think it's the best transmogging set for assassination/sub. For my combat rogue, I've put together a hodge-podge of gear that I like, but the most remarkable thing is that the belt and shoulders are from 30-40 level quest greens. No WWF belts or outrageous Joan Collins shoulders for my rogue.

I really like the simple offset shoulders that I found:

http://www.wowhead.com/item=33247

I found these single spaulders in four color combinations so I can always mix and match with other pieces if my transmogging tastes change.

"Rogues should be...well, rouge-like."

I'm not sure I can agree with this. Quite frankly, in PVP, a red rogue is a dead rogue. :P

On topic, I'm not sure what I want my armour sets to look like. These three paradigms you've listed all can work well, and can provide the art team with nice inspirations to draw from for each class.

But really, I just want it to look good. I'm well aware that what appeals to me is by no means representative of the community as a whole, or even anyone else for that matter! But for me, that's what it boils down to. You can have the coolest concept in the world, but if the execution is poor, those cool ideas have gone to waste, and we're stuck wearing ugly.

To be perfectly honest though, I'm pretty easy to please. I can usually see redeeming qualities in armour sets that have the general wow forum population ranting and raving (not that it takes much :P) I definitely hope the rise of transmogrification means that the art team can be a bit more adventurous with some of their future designs. I think DKs in particular could use a bit more variety in their tier sets. Skulls and spikes, anyone? There's gotta be more than that! :D

Agreed with the kilt's for Shamans and Paladins.

BTW, I think set design should be based on class iconic looks rather than the monsters we kill. That's what I liked about the Vanilla tier sets.
The best pally tiers for me are T2, T6 and T13. All of them were based on Paladin lore more than raid lore.