One of the things I've noticed about the way World of Warcraft is developed is that, as players reach the current level cap and wait for the next expansion to drop, a series of placeholder patches are released that include dungeons and raids and increasingly superior equipment. However, each of these patches tends to include nerfs to its immediate predecessor, so the game's difficulty curve isn't so much rising continuously as it is perpetually unrolling.
As someone who only got into the game with Cataclysm, I found the leveling experience so easy that I blew right through Outland and Northrend without even noticing their higher-level dungeons and raids—yet as far as content density is concerned, those areas are significantly "deeper" than the wading pools I spent most of my time in: after using the Dungeon Finder and questing in Borean Tundra and Sholazar Basin, I had barely scratched the surface of the Icecrown quests before it was time for me to move on to Cataclysm zones, yet the Argent Tournament and various Icecrown Citadel dungeons and raids could almost take as much time and energy as I spent on the rest of Northrend put together.
What other content-deep locations did my rocket-jump to 85 bypass? The fact that Zul'Aman was transformed from a raid into a dungeon suggests these wells are ripe for developer recycling, and I'd like to experience this content the original way (or, as close to original as is possible in the year 2011) before they get remixed and rereleased.