Unless you choose (like many players) to mindlessly grind yourself to top level so you can gank newcomers with impunity, there's not much to do except play that linear quest sequence.
Sometimes there might be several parallel sequences, but you're still not really a part of the environment.
A truely dynamic world would have player actions shape that world, cause things to happen and evolve in some way.
Very few mmos (or rpgs in general) have that.
Morrowind is one single player rpg that does it, Fallout 3 also (to some extent). In mmos, Ryzom is the one with probably the most actual interaction between players and environment (and environmental impact on players, with resources and wildlife being affected by weather, time of day, season, as well as interaction with them).
Ryzom also features some player influence on the development of the lore, with events being dynamic in that the outcome isn't fixed and can have influence on future events (I've not yet seen events where the outcome changed the world, though events tend to build up over days with event spawns starting to appear in places for people to discover even before the event is announced).