As others have said, a sizable land area with lots of zones. The zones themselves don't have to be huge-remember that Grizzly Hills was fairly small, and that zone was looked back as one of the best in Wrath. But I'd definitely like to see multiple zones per level. I hated the Cataclysm model, where the only choice was in the first zone, and after that you got funneled into the repetitive sequence of Deepholm, Uldum, and Twilight Highlands. Considering how terrible Uldum was, the fact that I had to level there was a major source of irritation. I'd have skipped it if I could've.
Also, and this should be said, zone diversity. MoP had a lot of zones that kind of blurred together in a way. If possible, it'd be better to make each one feel more distinct in the manner that Wrath and TBC did (cataclym's zones were distinct but also disjointed, which was another issue altogether).