Bwah (nice name) of Undermine asks, since we've already heard that flying mounts in Azeroth is not a possibility, if epic mounts around, say, 300% speed are. If we were able to zoom that fast across the landscape, it'd be almost like flying, and it would definitely make the experience of leveling faster. But Neth says no-- Blizzard wants the world to "feel" big, and making travel even faster would reduce that epic size of the landscape.
Unfortunately, that's a bit of a cheat-- slowing players down just to make the land "feel" bigger isn't as good as actually crafting a big land itself. Which Blizzard did, I think, and I doubt 300% mounts would "shrink" the land that much.
Of course, saying 300% mounts would make leveling go faster is a cheat, too-- sure, they'd be fun, but do we really need them? A better solution would be to work on making the content so good that we don't want it to go by faster in the first place.