Actually, I've been having problems with misdirect too, at least casting it on my pet. It seems to work fine on tanks I have focused in dungeons, but when I was doing the new Stranglethorn chain if I cast MD on my pet, with or without the macro, and then pulled a mob, the only threat on the mob was mine, the mob wold run right ot me, and I'd have to FD for my pet to pick it up. Even if I sent my pet in first to get initial aggro and THEN MD'd, none of the threat seemed to transfer and I would soon out-aggro my pet.
This!! This is exactly what was happening to me! It was in the STV area for the ZG questline so maybe it had something to do with that phased area??
It's working fine, as far as I can tell, when not in that area and when I do dungeons...