At equal level, if a Tank is losing aggro then either they suck at Tanking, or the Healer / Dps are being asshats. Pld's have the lowest enmity multiplier by far (Cuz' SE is stubborn and refuses to fix anything that is wrong with that job, for some reason), and even they can hold enmity if they use their rotation correctly. Drk's and War's shouldn't have any problems at all establishing and keeping enmity.

There are only 4 occasions in which a Tank should be losing enmity. 1) They're dead. Can't hold hate if you're dead. 2) The healer is a Troll and is doing stupid things like pre-casting Regen on you before each mob pulls. It's not the Tank's fault if the Healer doesn't know how over-healing enmity works. 3) The Dps are suicidal and are either pulling things on their own, power focusing your off targets, or splitting up and attacking separate targets from one another, therby dividing the Tank's attention. And 4) They are under-geared or out-geared by the Dps/Healer... All of these things can be worked around if the tank is paying attention, though. They just have to nerf their Dps/Mitigation for the sake of compensating for the team.

It's also noteworthy that a lot of the DF tanks people run into are not actually Tanks. They're Dps/Healers who got fed up with long que times and swapped to get their Roulette's done. No surprise that they don't know the job well when it's not their main.