Didn't really bother reading the past posts here but anyways here's my 2 cents.
I've played WoW around March of last year (on a PvP server) after my friend kept convincing me to get it for so long. It was a bit slow and boring until I got to around lvl 30 where I could do lower level instances, get better loot, go to more diverse places. From lvl 30 I would spend most of my days wasted on the computer I mean seriously during summer break ...oh boy it would be waking up at 9:30 to playing till 11 at the night. I just couldn't stop even though it was boring I just kept pushing myself to level and get to 60. I got to around lvl 56 on my paladin, a class who most people like to bag on because "they're so easy to play" says the masters. I pretty dominated in PvP against hordes and in most duels even against one of the most dominating 1v1 dual class in the game, mages (well for a paladin at least). After lvl 56, I felt going through same repetitive boring instances to get gear and lvl up and from what I hear from my lvl 60 friend, it only gets worse when you hit 60. This game just wasn't worth what I was missing out in life, friends, and my general real life interests and hobbies so I called in quits and I cancelled my subscription and evidently, deleted my character without regret.
Now don't get me wrong, this is a fun (but addictive) game and I honestly thought I had self control but this game sucks you in, and it doesn't try and let you go, not to forget the $15 monthly fee which I was soon getting pretty sick of. I'm not saying your going to play 10-15 hours a day if you get this game, alot of people can control how much they play this game but it's just a heads up. All in all, I can't complain about how well this game was made but just be sort of careful about where this game can lead you to...
Oh and if no one has answered your question yet, I'll summarize the different realms for you:
PvP: The most realistic experience you can get out of this game, a constant battle between the Horde and the Alliance and though you may regret and be sick of getting ganked by higher lvl enemies, you'll soon get in on the fun where your high enough to gank others.
PvE: Easiest place to level up to 60 without interruptions or hassle of running to your corpse from constant ganks. My opinion, you'll get pretty sick of it and wish you were in a PvP realm sooner or later.
RP: Oh boy ...this is where they "try" to give you the realistic experience with people trying to talk like their own races somehow (who knows?). Just visit the RP forums in WorldofWarcraft.com ...you'll see what I'm talking about
Boy that's a long review, I'm off to sleep now.