Another problem is probably that a lot of people do things differently in MMOs than they would in RPGs. In single player games you explore anyways because that's what the game is about and you have nobody around to pressure you. In MMOs however people are driven by success. They want to keep up with everyone else so they feel that they have to go out of their way to explore if they could instead grind dungeons, speed level etc.
In GW2, exploration is based around going to the locations that ANet think you should go to. You have them on your map and they get filled in when you visit them. Now, sure, there are other things to see, things that are not on the map. But since there are things on the map, the rest gets kind of lost in the noise.