Why does it need to be?
Before you stop and comment on how there's no permanent impact, you should first act if you need to have a permanent impact, and if you truly feel it does need to be permanent you should then ask if that's something that doesn't get in the way of the overall goal of the game (or other players), and if you still think that doesn't get in the way, then you need to ask if that's reasonable to program on the technical side.
To be fair here - let's consider that your actions in WoW don't have long term impacts any more than your actions in GW2 do. But at least in GW2, the world isn't static. That's the primary advantage here, and the main point behind DEs - not so much that your actions have impact, but that the world goes on... with you, or without. It doesn't revolve around you, after all.
---------- Post added 2012-04-27 at 10:35 AM ----------
Anyways, more on-topic, I found three videos that pertain to this subject:
Notes: The follow-up video is way off in a lot of places. Namely in how he doesn't understand that dungeons also have random loot. He also doesn't understand the each dungeon path will only drop specific item slots, and thus you have to run every path to get the dungeon sets. I didn't pay much attention to it because it rambles wildly all over the place, but it's fueled intensely by misinformation.