It's not even worth PvPing as a vengeance DH now, they were already insanely squishy in BG's, and this has just made it worse. I have a feeling this was put in to help tone them down in world pvp, but now they're just fodder in BG's. What goods a tank that can't carry a flag as well as a rogue?
My suggestions in how to make tanks more viable without being broken for PvP:
1. They're actually tanky, they can't just be blown up by a solo player in a stun.
2. Taunt actually does something untalented, forces the selected player to target the tank for 4 seconds. Given that taunts have an 8 second cd, this is a 50% uptime. Could be adjusted, taunt CD increased.
3. Rewarded for defending an objective, I don't mean the "you get more honor for killing people here." buff either, for each minute spent defending an objective, you're rewarded with bonus honor or a temporary buff.
4. Tank damage within PvP is reduced across the board, it's not a tanks job to DPS, it's their job to control the battlefield and take the damage for their allies.
5. Every tank is given an AoE taunt on a 45 second cooldown with a 20 yard range (See point 2) this taunt would last 6 seconds.
6. PvP talents that increase the damage a tank does are replaced with talents that either improve their taunts or provide additional CC, allowing them to control the battlefield.