You can't really do that in an MMO, because the world has to remain static. For instance if you do the Correllia world arc, win, and your faction takes over the planet you can't just have the mobs, quests, etc, just all go away for you. They still need to exist there.
You also do get a sense of what you've accomplished in the final scene the world arcs, plus the next chapter(s) often times explain what happened, ie what faction won what planet, and so on.
WoW did it ad-nauseum in Pandaria. While I am unhappy with how things look after the questing, I applaud its literary significance.
Alternatively, faction/reputation systems help a lot. Increase your standing with gangs in various planets' underworlds, planetary police forces, political campaigns, by doing jobs for them. Radiant quests work really well here.
Though there needs to be some sort of "incomparable" reward as well. Access to restricted areas is always fun, but the areas have to have some interesting qualities... I shall think much on this.