Three steps away from the level cap, I am astounded.
Looking back on all the questing I have done I realize now that Blizzard has held true to their promises. They wanted story to be a big focus this expansion and by golly they threw the whole nine yards into it. It's wonderful.
The Burning Crusade was, and remains, Blizzard's huge gameplay fix. For the most part they tore up the foundations of the game in order to rebuild it all better. The job they did was superb and the expansion reflected that. Burning Crusade was built to test the new gameplay mechanics, to require the playerbase to learn them, understand them and love them. For those who could it was awesome.
For everyone else, or those who were tied down by everyone else, the expansion was half-baked. Blizzard's big mistake was assuming that all players, or at least most of them, would eventually come around and learn the esoteric intricacies of their carefully designed game. They found out the hard way that this was just as huge a barrier to entry as the required overhead of organizing forty people. Huge swaths of content were left undiscovered by the vast majority of the population because they could not, or would not, min/max to the extreme levels required.
On top of that, there really wasn't a cohesive story to Outland. It was steeped in history, brimming with interesting peoples, and had many memorable bosses. However, none of it really held together. The Burning Legion opens up the Dark Portal, forcing the Horde and Alliance to work together and push them back. Upon doing so they... wander around a marsh, help some ogres, relive Jim Henson's Dark Crystal, fight alongside mana sheiks, scale Medivh's tower back in Azeroth and generally fail to encounter any centralized threat from the Burning Legion. Oh, and Illidan dies somewhere in there.
Wrath of the Lich King is quite the opposite. Upon landing in Northrend you're surrounded by Scourge forces. While there are quests along the way which divert from battling them, everywhere you go the touch of Arthas can be seen and felt. The quest text that was occasionally interesting but mostly ignored in the Burning Crusade carries far more character and depth. With the simplification of many game mechanics the whole thing is in some ways an about face from the Burning Crusade.
I am only now nearing the completion of the third of nine zones (ten if you count Wintergrasp). It is likely I'll only have completed four before I hit the level cap, but I intend on completing them all. Meanwhile my Death Knight languishes.