Even tho one might argue that the beginning of the instance with boring gray scenery and iron dwarves isnt visually impressive at all, once you get indoors you are just blown away. The amount of detail they put into most random stuff like the holograms on the walls when its just a minor corridor with nothing special to it apart from killing trash to move on is just mindblowing. The scenery just screams titan origins and they havent put so much effort making raid feel epic after ulduar.
The way you unlocked hardmodes was just brilliant. Something as simple as nuking XT's heart in one go or leaving the Ancient's alive on Freya to make the boss harder and the fight completely different was amazing. Really disappointed that blizzard has moved away from this kind of mechanic to introduce the hardmode to the current "hi lets click it to heroic" which is rather insulting towards the player base. You used to feel that you achieved something when you finally had enough dps and raid capable of nuking Xt's heart in one go rather than just toggling heroic on.
Alternative Progression Paths:
The freedom you felt in ulduar was amazing also, didnt feel like killing Razorscale first after Flame Leviathan? No problem you could do Ignis instead. Same with council and Kologarn not to forget keepers also. Rather than making raid this linear pipe you actually made it feel like you had choices to advance onwards in this massive titan made prison.
No clue why i wrote this random stuff, stopped serious raiding weeks after i got my legendary daggers in cataclysm. After that only done Lfr to see the current content once and quite frankly nothing comes close to what ulduar was.