Some tips for tanks from a BRD with aggro problems:
1) Please mark. I know this doesn't fully prevent my less observant brethren from attacking random crap. You (and I) are hoping, though, that it at least keeps the decent/good DD out there doing what they should be. Nowadays, I mark, but before I was having issues even using Target of Target, because apparently I kept catching the tank as he was rotating through to hit second/third mobs. At that point, I'm going to pull aggro. I'm sorry, but I can't un-fire that Windbiter at the low-priority mob.
2) Please keep things off of the healer. So far, my healer has almost always been in Mumble with me. If he says he has aggro, I immediately check to see if the tank has noticed. If not, I'm popping cooldowns to try to pull off of the healer myself. If I see the tank Provoke off of whichever of us has aggro, I'll switch back to the main target; if not, it's a race to see if the mob or I dies first. There's only so much I can do, though.
3) When moving out of AOEs, please don't run towards the healer. If you can help it, don't run to the DD, either, but especially not the healer.
For DDs out there:
1) Please don't get ahead of the tank. Running ahead a little can be okay if, and only if, you know for a fact the way is clear. Attacking ahead of the tank is almost always a horrible idea. Don't do it.
2) If there are marks (there should be), please obey them. Don't AOE pulls that aren't meant to be AOE'd, don't break CC if your party uses it, and don't attack the #2 mark when the #1 mark is still at half-health. If you're pulling hate, scale your attacks back a bit. Stop popping cooldowns if your tank is having issues keeping things off of you (assuming you're attacking the right target). Don't unload straight into a mob as soon as the fight starts. Especially if it's a boss.
That said, I'd say I've only met a few great tanks in DF, a fair number of good tanks, and more bad ones than I'd care to remember. Anything good or above is enough to finish the dungeon, though, so who cares?