I have played every tank, blood DK is the last one I've levelled (at level 52 thus far) and I think, my favorite.
It's really fun.
We have a shitload of cooldowns for every situation.
Taking lots of physical damage? Icebound fortitude for 30% flat damage reduction for 10 seconds (and no stuns), and Rune tap for another 20% which you can use twice. Dancing rune weapon when necessary for even more protection. Magic damage? Anti magic shell for a big absorb, Anti magic zone for another 20% reduction. Also can summon a ghoul and sacrifice it for another 25% hp boost.
That's not even including the self healing. Pop Vampiric blood and a couple Blood boils and Death strikes and you're back to 100% hp. With Will of the necropolis we take 30% less damage when under 30% hp. Once you're that low a couple self heals and you're good to go. I love the synergy between Bonestorm and Swarming mists (venthyr ability). Get to max runic power, hit Bonestorm to get your healing up, then Swarming mists to get your runic power back up. Best of both worlds. It's also a good times to Door of shadows into a group of mobs to take them on.
Then there's the taunts and interrupts. We have 2 taunts, Dark command and Death grip (awesome for casters). And 3 interrupts which are all RANGED! Death grip can interrupt, along with Mind freeze and Asphyxiate. All can be done from 15 yards out.
Most agree we're the least mobile tank, but are we really? Death's advance increases our movement speed by 35% for 10 seconds and during this time nothing can drop our movement speed to less than 100%. Only a 45sec cooldown. Grip of the dead might be the best snare in the game, enemies in our Death and decay are slowed by 90%. We also have Chains of ice to slow individual targets by 70%.
All in all having a lot of fun playing Blood DK. I only play casually so I can't comment on high end content like mythic +15 but for me it's really fun so far.
Hope you are all enjoying it as well!