Having been at lvl 70 for a while and taken my well documented detour and time gearing up, and tanking lots of Heroics I can safely say my opinion of Karazhan now that I've been there 3-4 times now.
I've spent a lot of time on the rep grind & faction grind for all my gear and tanking Heroics with decent gear. Now that I'm raiding Karazhan this is what seems more than obvious to me in my own opinion.
Mobs in Heroics hit much damn harder than in Karazhan. Allot harder in fact they hit in Heroics, but thats my honest opinion there and I'm sticking to it. Trash mobs in Karazhan may sometime do some wired tricks but running 5 man Heroics is harder. This now reminds me of when in Heroics often times that I get some Kara raiding healers with epic gear does not often realize how much harder mobs hit in Heroics compared to Kara. They may not anticipate just how hard mobs hit a Tank in Heroics and I've had many such healers that have claimed because they raid heal in Kara they can heal just fine in Heroics. Not all of them do. However the point is in my opinion as a Tank. Trash Mobs in Heroics hit much harder and can put a royal pounding on Armor.