I have a guardian druid 880 ilvl , a DH 874 ilvl and have had lots of experience in healing all tanks. In this post I want to clarify the flaws behind DH/DK tanking and see what could be done to improve on those classes. Let's first define what a "tank" class means. It's obviously a classs much more durable than any other class which can be on the front line and take the punishment while being healed and while DPS are bringing the target down. All tank classes are quite durable in the sense that they have high health, armor, reduction abilities but the issue is that some of these (DH,DK) are designed around sustainability rather than durability. What I mean is the following : A druid/warrior/paladin for example are sturdy classes able to take a beating and then receive healing to recover while DHs/ DKs are designed around taking A MUCH BIGGER BEATING for the cost of recovering easier.
The problem is that these 3 classes are PROactive tanks while DK/DH are REactive tanks. Which one do you think is the better mechanic? Take very little beating and slowly recover or take a huge beating and then pop back to full? I can tell you right away... the 1st one is the better case because when you are PROactive you do what you are supposed to and you know that there is no coming back and you've done your job. Your mitigation is ON and you just wait for the hit and move on while being REactive means that you must first take the big beating and only then (if you are still alive) you can make use of your tools to heal up by death strike,soul cleave, etc. Ok so lets have the scenario:
You are in a raid healing a warrior and a DH. You see the warrior's hp going down slowly and you know that you must heal it. It comes down gradually and it goes up gradually because of your heals. It's predictable and there is smoothness. You know that you will do it properly as long as you heal consistently enough. At the same time you see a DH tank taking a huge hit and dropping from 100% to 40%. You throw a couple of large heals to the DH but it turns out he has just used his 5-point soul cleave and there's been a ridiculous amount of overhealing. What do you think about that? How do you feel when you see the DH hp topped just before your heals land and you make 2 million overheal? Thats utterly WRONG! Healers never know when the tank will land a cleave or a death strike.... It makes healers life much harder and much more unpleasant. Then in 15 seconds you see the DH dropping to low again but you tell yourself that you ain't gonna do that mistake again and waste those heals. He seems to have not enough souls up to do a proper soul cleave and he takes 2 more hits and dies a miserable death.
I am saying all of that from the perspective of having done all raids in heroic as a tank on my DH and druid and having healed all tanking classes as a druid on heroic difficulty.
All of this results in Bears >Warriors > paladins being the best tanking classes by a wide margin. Logs confirm it, top guilds confirm it, tanking experience and healer experience confirms it and most of all and most importantly MECHANICS / class philosophy confirms it.