That one big mistake that I see often in long-term MMO's, the mistake that really kills the game for me is when the dev's decide to create some central hub that everyone ports to and from. This streamlining of the game makes overland travel and much of the game world moot, and empty.
With EQ, it was the Plane of Knowledge that did this. With WOW, it was Dalaran and is now the garrison. Other games do this too.
An MMO's strongest feature is the world. So, why create ways for players to bypass it? The low level player having a tough time with a low level encounter should, on occasion, be bailed by the high level player who was passing through. That kind of stuff is great. However, riding around in circles in some central hub while awaiting dungeon finder to pop ... is not great.
The world is the foundation of a healthy MMO. The community is the cement.