Although WoW is by no means the game for me, what you are saying there is totally different.
The 3 big instances you are referring to, I assume you are meaning the 3 continents. Each area in each continent is in no way acting like an instance in itself.
If my friend John and I are both in the barrens, I can see John (providing we are close enough of course). I do not have the situation where I can be stood right next to the same building as John, but not be able to see him because he is in another instance. If I stand on the border of Westfall looking into the next area, I can see John approaching in the next area.
That cannot happen in AoC. Its got nothing to do with loading screens, loading screens are inherent of zoning, not necessarily instancing. In no way do the individual areas of WoW "act like an instance". In fact even the continents are not instanced, they are zoned. No matter how many people are in a continent on WoW, you only have one version of that continent (barring obviously the different versions that each server has).
If it helps, DDO, CoH and AoC are examples of an instanced gameworld. EQ2, LOTRO and WoW (to a degree) are examples of a zonal gameworld. UO and AC are examples of an open world gameworld.