I want to say something about population.
I played on Nostalrius last year. I loved playing vanilla again and enjoy a lively world with my favorite activity : hanging out in the zones doing some blend of solo PvE and PvP.
For me open world PvP / dueling is the most fun I can have with WoW / a MMO. I pretty much don't care about any instances, because I'm interested in the open world, unpredictable encounters, choices of engagement or not, varying parameters with involvement of different mobs and whatnot.
But Nostalrius was very, very crowded.
This wouldn't necessarily be a problem for any MMO, but imo it is for World of Warcraft. I would say mainly because this game has pretty crazy strong crowd control and some imbalanced things like invisibility, power outclassing with a few levels or some heavy gear. It's really hard to disengage from a fight. Many times, if you see someone 30m ahead and you're at 50% hp it's already too late for escape. If the world is crowded, you get ganked all the time in the most stupid ways, and it gets harder to find interesting engagements because there is always a bunch of guys nearby who will wreck you in 5sec if you attack someone of their faction.
In my experience, the World pvp situations work best when there is enough room to hide, locate, choose to engage or not, space to run away, to be left alone, etc. Such that a lowish or "medium" population for a pvp wow server seems to be the best to me, and overcrowded a potentially unplayable. I left Nostalrius for this reason. Stranglethorn was stupid. Even in Desolace, though it was possible to do some leveling, engaging people was way too risky because you'd quickly get caught in a 2v1.
Just some thoughts. Imo if you open a PvP server, that's great, but would it better to open a few? Think of how hard it is to escape when you are attacked, you really need the space. My best memories I think are still from the vanilla blizzard servers with a "recommended" or "medium" population. I guess even the "high" were most of the time not nearly as crowded as Nostalrius?
The past weeks, I've been playing on a server with a rly low population. It is too low.. but it is more enjoyable. But it really is too low... Due to the way the game is designed and the size of the world, population is really important for a WoW server, especially PvP.
[if you ask me I think this game could be 5 times better if you tone down the crowd control, some of the charges or bursts, invisibility, bit less power difference per levels, and add some things to do in the world

it would be amazing ; make more zones matter to more levels and easier to disengage]