I don't play WoW anymore, but I will give my insight on it seeing how I started playing in January of 2006.
After having three 70s (Paladin, Rogue, Priest, respectively), I came to the conclusion that the only thing worth in the game is end-game raiding and that's it. Pvp is a nice perk and doing daily quests gets you money, but it's not as self-gratifying as raiding. Once you're done raiding (and that's by getting your character 5/8 or 8/8 T6), there's pretty much nothing else to do. Yes, I'll be going back in WotLK just to level my characters up to 80.. who knows, I may reroll a new class before then, my record getting 1-70 is 10 days.
This jibes with my experience too. If you enjoy the end-game raiding and you have guildies who's company you enjoy then WoW has incredibly mileage. I did the hardcore raid thing for about a year and a half, all through the vanilla WoW content, and then all the pre-Sunwell TBC stuff and had a blast, but the content really isn't enough to keep you going on it's own. I lost interest pretty quickly after my guild did the inevitable giant drama-bomb/falling out. (Should probably be happy it lasted that long, lord knows we saw enough other guilds flame out spectacularly)
@MeowMix You need a 55, not an 80, so most people who resub for WoTLK should be able to roll DKs on day one.












