Yes Valk, just... yes
I was talking about EQ today, for like 2 hours, and the comparison to MMOs of today. That living, breathing world is definitely very important. I mentioned Plane of Hate, (and referring to the original version) this zone was designed with the denizens of hate in mind only. How will you attack this plane? I don't care, it's their home and it will cater to them. It certainly was hard, yes, but you really got a sense that you were invading their home. Compared to zones now that have clear entry points, maybe you can leisurely pick up some quests while you buff up and summon people in. Now there's nothing terribly wrong with this, and WoW does do it the best, but it really seems to detract from the whole invading an enemy's home feeling, and replacing it with the feeling of just doing a loot run.
If anything should be kept in mind, it is that designing a dungeon should cater 99% to the denizens' needs within it. It should have everything they need to survive. It should appear to give them a clear advantage, since you are in their territory. I believe there is a good article on dungeon building on, actually, mudconnect.com. All these well designed dungeons and areas contribute to the living worlds that should be and surely there are other factors, but this stands out as extra important to me.