I started in late TBC, and by the time my first character hit level 50, I was growing really bored with the leveling process. I was about about nine and a half /days into it, and I increasingly felt like I didn't know where I was supposed to go or what I was supposed to be doing. An open sandbox is great, but there needs to be something like the Warchief's Command Board to allow people to find their way back to the main storyline (or in the case of an MMO, direct them toward level-appropriate content).

I can see how single-toon folks might be upset that time was used on the 1-60 experience, but it needed work. Remember how Hyjal and Twilight Highlads ended? You beat Ragnaros/Skullcrusher and there's much rejoicing, and then... everybody just stands there. There's nothing to tell you "This is the end of the zone, go back to Orgrimar and start the next episode." Vanilla zones had a lot of those "okay, now what?" moments, and where you did get pointer quests, they were sometimes obsolete by the time you were ready to move on.

But then how much of the 1-60 questing will make any sense a year or two from now? Deathwing--who's Deathwing? Didn't they kill him a couple expansions ago?

At least by keeping BC content confined to the Outlands and Wrath content confined to Northrend, you know you're going back in time to deal with an isolated menace. Not so much if SW is still torn up when pandas are bouncing around.