Dramatized has a simple but intriguing question on the forums: "Is Azeroth flat or round?" It's got to be round, right? You can prove that the world is round a number of different ways, but do they actually work? I don't ever remember seeing ships disappearing beyond the horizon (in fact, Tydeus confirms that ships don't drop below the horizon, so either Azeroth is flat or just really, really big), and in most places on Azeroth, the horizon is actually a mountain range. And I've never seen an eclipse on Azeroth, so we have no idea what shape the world really is. The ingame map between Azeroth and Outland hints that it may be a globe, but really it's just the map overlaid into a circle -- no hints there.
Neth hints in the thread that maybe even the Explorers' League doesn't really know if the world is round or not. But whether it's round or flat, it's a pretty good guess that there's a lot to this world we haven't yet seen -- if it is flat, neither the Horde or Alliance have reached the edge yet, and if it's round, you'd wonder why someone hasn't sailed from Darnassus across to Azshara (and if Northrend is what's north of the two main continents, what's south?). Remember this the next time someone asks just what else Blizzard can visit in future expansions -- there's a whole world out there we haven't been able to see yet.