During the end of TBC i was in a guild doing Sunwell. Wiping nights after nights on M'uru and Kil'jaeden (before THE patch). Well for some that's obviously not so much fun. But it was a challenge that kept us playing.
After three month WoW break i came back and took a step away from hardcore raiding (4-5 nights a week) and went back in my old more "casual" raiding guild. I remember bosses in SSC/TK/BT/Hyjal that took at least several nights to accomplish.
After 2 weeks of 25 man raids now we cleared Naxx yesterday. Some ppl have seen Kel'Thuzad for the first time, some downed him in the 10man version. First attempt was (sadly) a 1-2% wipe, second attempt we got him down. Of course for some it's fun to just go there and grab loot. But what's the reason if you don't really need the gear for this content.
That's something that just takes some fun from raiding right now. Besides the instance being rather old. There are ppl who raided in vanilla WoW Naxx all week, i can't think how bored they feel right now
Besides. At least we have some more challenging tasks like Sarth 2/3 or 3/3 in 10 and 25man versions, thats actually something you do not accomplish on the first try.