It seems to me that the Warcraft universe does seem to have gravity similar to our universe, at least as far as falling and flying go. But I agree that it's possible that gravity on the planetary scale works differently, and maybe the answer is just "magic."
It's a super minor thing, but Azeroth the planet is in the Great Dark Beyond. The Twisting Nether is an "astral dimension" that exists "outside the borders of the physical universe," according to Chronicles 1:1.