IMO, open world vs. forced zones makes a HUGE difference. I personally see walled-off zones as a way to cheat to make the world seem bigger, forcing you to stay within the bounds of a zone to make it from one end to another. Consider how long it takes to go from the Barrens to Tanaris in World of Warcraft on foot, and then consider how long it would take if all of the mountains and cliffs in Thousand Needles had shortcuts and passage ways, allowing free travel to neighboring "zones" without having to take a pre-determined path.
In addition, travel speed plays a huge role in how big a game's map feels. In Asheron's Call, for example, the map was massive. It could literally take a few hours to run from coast to coast, and run speed at moderate to high levels in that game were easily equivalent to 100% or even 150% movement speed in WoW. Riding at 100% speed (epic ground mount) in WoW, on the other hand, would take you from one end of Kalimdor to the other in a matter of what, 30 minutes? And that's even with zones forcing you to take a less-than-ideal path.
With the way that they designed WoW, I think that adding flying mounts was a huge mistake. The world doesn't even feel remotely big now, mainly because you can simply fly over everything. In Asheron's Call, on the other hand, it would have been "nice" to be able to fly over a few mountain ranges or shear cliffs from time to time, but ultimately it wouldn't have made a big difference in travel time because of the world's open nature. As others have said, AC didn't have a bunch of landmass filled with nothing; there was something to see or do on virtually every square inch of that place.