Burning Crusade didn't only attempt to improve questing, but dungeons and raiding as well. A big part of this was the new Heroic difficulty of dungeons. Rather than have an extremely small set of endgame dungeons, every Burning Crusade dungeon could now function as end game content!
A great idea on paper, in practice there were issues with the implementation. Not all Heroics were created equally in difficulty or loot distribution, which essentially halved the number that were practical to run. Some Heroics were harder to get access to than others as well. The result was that the planned progression of Normal->Heroic->Raid was a rocky path with a few stumbling blocks.
Nevertheless the progression path was still functional, and the act of progressing was itself a means by which player excitement was stoked. Each surmounted challenge was an accomplishment, and as players overcame each one they felt that budding thrill as they stepped nearer and nearer to being raid-ready.
This thrill continued as players progressed through each raiding tier. Though, again, the logistics of shifting from 10-mans to 25-mans was another unfortunate implementation issue, the thrill of finally being good enough to enter and acquire gear from Serpentshrine Cavern or The Eye was real and powerful.
Where I feel Blizzard has erred, perhaps irrevocably, is in demolishing this sense of preparation and accomplishment. This first started in Wrath of the Lich King, as the revamped badge system essentially made old raiding tiers obsolete as new tiers were released. The problem has expanded in Warlords of Draenor as running dungeons, even Heroic dungeons, has become strictly optional; within a day of reaching the maximum level I had sufficient gear to queue for raids in the Looking for Raid system. The feelings of anticipation and accomplishment for simply walking into a raid instance are gone.
Bringing them back into WoW isn't something that can be done easily, but as I play through Burning Crusade's content one last time I can't help but wish for those feelings to return. They needn't be reserved only for those raiders who challenge the new Mystic difficulty for raids.