Okay guys. After spending days thinking this over in the back of my mind and trying to figure out which sock drawer Blizzard stuffed the "epic" into, I've come to a conclusion: It's not about the "epic". What it is about, what's lacking, is something that's strictly based on design. Let's go back to Ragnaros and Molten Core for a moment and see if I can get this point across: the reason that Ragnaros felt "epic" was because the dungeon itself was specifically designed with 40 players in mind. The spacing of the zone, the placement of the rocks and bosses was all designed around the idea that there would be 40 players in this zone.
Moving on, Hyjal felt odd at first because I was used to that 40man model. It faded because the dungeons of Burning Crusade were designed with 25 players in mind. Hyjal, Black Temple, Serpentshrine Cavern, all of it, designed with the intent of 25 players being present in that zone, so they felt natural. On the same principle, the 10man dungeons -- Karazhan and Zul'Aman -- both felt exactly right, because they were designed with 10 players in mind. Karazhan was huge, but not once did the experience feel awkward because all boss encounters and rooms were designed around 10 people playing in there.
When you get to Wrath, Ulduar in particular -- Ulduar was designed with 25 players in mind. The boss rooms, the open spaces were all created specifically so that 25 people would feel like this space was absolutely gigantic. But when you take 10 people in that space, what was once comfortably "huge" for 25 borders on the absurd for 10 players. That's why it doesn't feel right -- because the space simply doesn't fit the people in it.