From a PvP prospective, I'm surprised that Outland is having trouble sustaining players while Wrath is keeping a constant 12000+ population.
I've played both servers for a while, and the problem with PvP on Wrath is over-geared people are Gods. They can run into Gargetzan, and kill every single guard no problem. In AV, I had many BGs were one side ran over the other side, and destroyed the guards in the Tunnel to GYcamp there. Also, the bosses got nuked HARD, they are not suppose to be beat by only a few players so fast.. Wrath was not designed to have people on each side with Shadowmournes running in and obliterating everything in seconds, its not good design.
On the TBC, you simply could not kill every guard in Gargetzan or get into the tunnel in AV because the guards were strong nor almost singly handily destroy a mini-boss. Gear scaled much less, and everyone was more on even footing. There was much more counter-play, bursting didn't happen all the time. It felt like I loss battles from being CCed on good timings, disarmed, and simply being outplayed.
My prediction is people like to win more than having a good battle. People want to be god with SM and destroy everything in their path and be declared the team hero than being average in a good battle. That's what irritates me about Wrath players.
Due to Outland dying, I decided to try out Wrath. What's laughable is the 80s here act victorious and proud of themselves over slaying a smaller fish than them. It's like the kids that are bad at video games, and retreat to killing low levels to feel proud.
In short tho the major problem with Wrath is everything is too bursty. I'm pretty much already dead before I react in most situations; it doesn't feel natural or healthy.