-last raid to feature old difficulty system (10+25) while fully occupying a raider's week (ToC onward with 4 difficulties)
-first and last raid to fully embrace the hardmode system within the same zone difficulty (disregarding sartharion here)
-entirely non-linear progression path (minor exception algalon)
-first raid to feature a hidden endboss (continued in cata and mop)
-most fights were mechanically innovative (FL sucked tho) and difficult
-extremely lore heavy raid with very memorable and grand endbosses
-big variety of scenery, amazing voices and overall simply beatiful and huge
-big sense of reward upon progression (1 hm item drop per boss made them feel very valuable)
-featured few items that incentivize visits even in late toc/icc: comets trail rogues deep into icc, flare of the heavens bis for all casters except shadow priest until reign heroic from anub, mjolnir runestone until late icc for rog/ware,conductive seal shadow priest multitarget bis until icc)
yeah, in my opinion, ulduar is without a doubt the most beatiful and from a raider's perspective satisfying piece of content blizzard ever created. for its era, it is unmatched. they took big risks, some of which backfired, and to be completely honest I fully understand and respect that very casual guilds most likely didnt have the greatest time in there, toc came out before most casual guilds have seen yogg25 normal, let alone any hardmode so it's understandable it might not have been a commercial success for the broad audience.
I, however, will forever remember it as my most enjoyable raiding experience