I have always been a fan of the idea that it is possible for an MMORPG game world to have a living feel to it. Player-driven content is always a touchy subject, but if it can be enacted on a large scale, I think it could be a very cool feature that would set this game apart from all of the rest. I understand that major settlements and canonical areas need to be static to some extent, this is just the nature of the beast when you have something shared by thousands of players.
But picture a more remote area, within which it is rumored that there may be ruins, treasure, powerful meanies that need slaying, etc. In games such as these, areas like this are sure to attract dozens of adventurers looking to make their mark on the world or strike it rich.
Perhaps after several days or weeks of gameplay with travelers making their way to one of these places that is so far out of the way, the wilderness route to and fro begins to resemble that of a well-traveled pathway. More time passes and the area becomes more known as an adventuring hotspot, and a small camp with some rudimentary comforts is set up by some NPCs. A few more days of gameplay and perhaps an AI wandering caravan appears, looking to sell their wares to these adventurers, and, provided business is good, they choose to erect a small shop, perhaps an inn. Eventually you have a small town, and as the area becomes more heavily trafficked, the rate of wandering monster spawns in the immediate vicinity decreases, perhaps the town employs a guard to keep such things at bay. Who knows where things may go from here.
Such a player-centered community may eventually turn into a thriving beacon of civilization in the midst of a wilderness area. Will it stay stable, or will the local baddies take offense and attempt to burn it to the ground as a warning to the encroaching players' race(s)?
When players lose interest, and traffic dies down, will the settlement begin to die a slow death, eventually turning into a ghost town, then unrecognizable ruin as the wilderness reclaims its own? Will some players choose to build their player housing (should that be a feature included in this game) in the immediate vicinity, offering the community a sort of permanent protection from wasting away?
I apologize that I am rambling, but I see many other people putting forth their ideas as to what they would like to see in an MMO, and while I realize that having an impact on the game world as far as storylines and defeating the bad guys go is next to impossible and unfair to those players who do not "get there first" so to speak, I do feel that allowing the player base to have a lasting effect will generate the feel that this is truly their world, and create an attachment to the game that seems to be precisely what the dime-a-dozen MMORPGs that have been released over the past few years have lacked. Give the players a world that feels like their characters' home, and I believe that your game will have an excellent future.