Mind you I am primarily a DPS kinda guy. If it's a mob you are not targeting that you lose, probably the DPS' fault. Make it your mission to teach people how to use the Target's Target function (default is the F key). Example, I'm on my DK (can peak at 16-18k in random heroics). I set you, the tank, as my focus. The group is pulled and you establish a fighting location, this is when I hit the F Key and get at it. As long as you stick to that target, I can go all out and not pull threat.
Hell I use the F key so much that often (read being uber lazy), after the first target in a group pull dies, I'll simply hit TAB, F, F. The TAB selects something in front of me, the first F should bounce my target to the tank and the second F puts me on the mob the tank is targeting.