I cant imagine how this would be possible in an MMO. I mean, one persons actions effecting the game for thousands?
The closest I've seen is server wide objectives. Horizons used to have all sorts of building projects, like bridges and tunnels, that required teams of crafters to complete. Once they finished, a new area would open up. LotR Online recently did a similar thing to unlock a new area as well. Also, LotR Online has some separation of areas to give an illusion of making an impact. For example, you start off in a vibrant town, but during the prologue, the town burns down, and once you enter the full game world it is still a burnt ruin. Also, the members of the fellowship are behind certain doors that you can only enter at certain stages, giving the impression that they have moved on at the appropriate time.
It's all illusionary though. I don't know how they could really feasibly allow each player a sense that they are accomplishing something unique to their world.
Thats a good point, the creation of things. WoW took that approach a few months ago when they opened up a new Blood Elf area, players would complete certain quests to "liberate" the new town and open up its vendors. It obviously couldnt work wrt destroying a boss/structure/dungeon, etc...