The Burning Crusade, and in a recent interview posted on their official page, producer J. Allen Brack and art director Chris Robinson lay out exactly which of these lessons they're following through on for the new Sunwell Plateau instances and Wrath of the Lich King further down the road. They talk about how Sunwell is really the pinnacle of the Blood Elf design style and how players will see many design parallels with their starting zone.
As for WotLK, one of the cooler design changes they mentioned is more closely integrating the main story-arc into the leveling experience. Brack mentions how with WotLK, Arthas will play a more visible role in the lives of players, even well before they ever reach the level 80 cap. Similarly, he also discusses how the element of snow in the expansion is being used as much as a story-telling device as an aesthetic one, trying to get players to feel the isolation and despair inherent in the Northrend zone. It's pretty fascinating, as both Brack and Robinson are more forthcoming than usual, perhaps because it's an in-house interview.