I myself would love to see another Karazhan back during TBC that was one awesome raid, it was not not overly hard nor was it overly easy.. It was just right the most fun had was Opera, Nightbane, The Curator, the dreaded Shade of Aran very tiny room with 10 folks in it lol.. Then Netherspite and the best of all Prince Malchezaar that fight was cool the fun bit trying to watch where the meteors were going and not get toasted..
On of the more a pain in the rear bosses was Terestian Illhoof and Chess event but really all the bosses in it were fun to do and because it was tuned for 10 man is what made it good, not like some others trying to tune the same encounters for 10 and 25..