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In the near future i personally believe that most mmos will ha a seemles world, with no loading times.
but, there will be instancing and phasing, but also the loading to these parts of the game will be in the background, this will be so smooth that it will feel totally naturall and in most places there will be other players around, in other parts you will be alone or with your group ready for a special mission.
All in all, there will be an open world feeling with the advantages of instancing and phasing..
Best MMO experiences : EQ(PvE), DAoC(PvP), WoW(total package) LOTRO (worldfeel) GW2 (Artstyle and animations and worlddesign) SWTOR (Story immersion) TSW (story) ESO (character advancement)