Why is WOW raiding less fun now?

Liore on why BC-era raiding and Ulduar were more fun for her than recent raids have been. Partly it’s the progression model changing (the ruthless dumping of any content that isn’t the current tier into irrelevancy) and partly it’s tedious heroic modes (everyone but Blizzard agrees that Ulduar’s model was way more fun & interesting).

Two raids that always stand out for people: Karazhan and Ulduar. Why? What have they got that all the other raids don’t? What did the BC-era progression model have that worked?