Good level design is a closely guarded secret, but you can break-down and study all the great levels yourself. However, some really interesting stuff can be found on the World Creation chapter of the World of Warcraft: Behind the Scenes DVD. Chris Metzen starts by talking about how important it was for them to get the trees to feel right before they could really do anything else. It is pretty fascinating.

A note somewhat related to level design that I am always reminded of is OldManMurray's Time to Crate review system for games: http://www.oldmanmurray.com/features/39.html