Honestly out of the four raids in WotLK, I felt Ulduar had the worst encounter design. And not by a small margin, either. It was basically a ranged caster love fest where melee were relegated to interrupt bots and waterboys for their finger-twiddling masters. Didn't help that melee not named "Death Knight" were borderline if not totally shit tier in terms of class balance at the time (Arms was more viable than Fury... let that sink in for a moment), oh and DKs were also the only tanks because every other fight was "OK, soak this massive magic damage on a fixed timer!"
That said, I was willing to forgive a lot of its shortcomings because the art was really top-notch. Lots of unique boss models, "Steampunk TRON" architecture, items made out of stars (well, not as cool as the ones in Guild Wars). Some items that are still NPC-only that I really wish I could get, like the 2H sword the Vrykul guys use. Also gave us the first Voltron reference in WoW and the "I REMEMBER YOU... IN THE MOUNTAINS..." meme, good stuff.