I've been playing on Outland for 3 years now, and it's a well balanced server.
It has much PVP, And Alot of PvE.
I'm speaking from the Alliance point of view mostly now because i've been lvling a horde alt for a week now ;p
But from alliance side, If you go to a capital there is plenty of offers to raids or do PVP, Tradechat gets spammed with like 5 messages a second, so theres plenty to do.
Horde side has that less, and it can take a lil longer then the alliance side.
WPvP happens alot on horde side, attacking capitals everyday and towns like goldshire, just for the fun.
Alliance does WPvP aswell, but much less then alliance, they are more defensive.
AH at both sides is great, perfect to live in, if you need something it's right at the AH
(And normal price, not insane expensive.)
At outland at night there is a Queue lately again, that can go up over 1000 people.
For the rest, i came basicly for the same reasons to Outland and 3 years later im still there, so ye it's a nice server to play in.