EDIT: Just noticed you're not 60 yet. The answer is to keep your gear up-to-date and pull slightly more than you think you can, use cooldowns and spam Unleash. You're going to want WAR to level 2 (yes really) for Foresight and PLD to 22 for Provoke, but more so for Convalesence. At 35 you'll get Blood Price which you can use to generate rediculous amounts of mana to fuel your AoE spam. As a tank, your primary job is to keep agro, if you need to empty your entire mana bar into Unleash spam to do that, then that's what you do. While leveling, I'd say absolute minimum of one unleash per target then start tabbing through targets and hitting them with your combos. If you lsoe agro on groups, add more Unleash spam.
How do I deal with DPS pulling things before me? How do I deal with DPS deliberately targetting things that I'm not targetting and trying to pull things off me? It's simple my friend. I'll explain with an example of the dungeon I just ran for my 60 roulette; Aetherochemical Research Facility.
Firstly, you wait for buffs. The first pull (in ARF) is probably the most damage you're going to have to deal with for the whole dungeon, get your Protect. Then as Lyth says above, you hit sprint. Mages can't cast much while they're running and melee wont hit sprint because they need that TP. But as a DRK, you only need a small amount of TP to kill things, and your initial agro will be gained with MP. Then you run through every mob you can get to and you hit them all with Unmend or Unleash while you're running. This is important because sometimes, you'll be with some wise-guy DPS who wants to attack things while you're still trying to round things up or your healer will get nervous and throw you a heal mid-pull. Hit. Everything. Once you've hit everything and you're running to that first wall-trap thingy, hit some defensive cooldowns, I typically start with Shadowskin because it lasts for a while longer than Shadow Wall and doesn't require Dark Arts (which you might not have mana for yet). Then, once you get to the wall-trap thingy, you hit Abyssal Drain... a lot. You mash the ever-living day-lights out of that bad-boy until you're about to run out of mana (or like I do, when you've still got 2 in the tank). Then you hit Blood Price and use the infinite mana that this generates (assuming Holy is either not being cast or no-longer stunning things) to spam MOAR Abyssal Drain. At this point, you can stare at the DPS right in the face and dare them to pull something off you. If Blood Price wears off and you're not at full mana with a pile of corpses and gil-gained messages, get better DPS.
So to sumarise:
1. You stop the DPS from pulling for you, by sprinting to get there first so you can pull it and by making sure that there isn't anything left that they CAN pull.
2. You keep yourself alive through this with liberal use of defensive cooldowns.
3. You stop the DPS from being able to pull things off of you by targetting everything and by getting hit lots to generate infinite mana (for 20 seconds) so that you can continue hitting everything. If you're a WAR, use Vengeance for agro. If you're a PLD, you either dump your entire mana bar into Flashing, then Riot Blade to regen and repeat... or you just cry in the corner.
4. You win.