Engage the enemy before the rest of your group and you will get initial aggro. Then taunt big baddie(s), chokedown smaller baddies with talons and chain ranged baddies in.
Your job isn't to taunt everything around you all the time. Keep the aggro of the hard hitters and keep enemies together and preferrably away from the group, and you've done a good job

You can try to use your other class skills to get the attention of more mobs when you're tanking the trash. When you are tanking bosses, mobs don't really need to be taunted in most cases.

In a trash fight, however, you need to taunt the ''biggest guys'', as Jciampi wrote above. Let me give you an example. If you're playing a Dragonknight tank, you can first use ranged taunt (Inner Fire from the Undaunted skill line) on the big mob. Then, when the pack starts coming towards the group and there are some stationary mobs like archers or mages, you can use you pull skill - Fiery Grip from the Ardent Flame skill line - to pull a few of them and get them all together. Once they are nicely positioned around you and your group, use Dark Talons from the Draconic Power skill line to immobilize them ( if you morph it into Choking Talons, then all the mobs affected would deal less damage to you and your allies).

Other classes don't really have such an amazing pull like DK does. But, another example. If you are tanking on a Sorcerer, you can hit further away mobs with Liquid Lightning from the Storm Calling skill line (I usually have it on my Overload bar, since it's not really necessary skill for a sorc tank but just nice to have at times), its an AOE ground target skill. Then you just wait for them to come and poke the biggest ones with your melee taunt Pretty much anything that aggroes mobs can serve you well.

But I must say, having an AOE taunt in the game would be interesting.

You really do not need an AOE taunt, if anyone in your group is having trouble with large trash pulls, it is more likely a group problem (most likely with the dps) than a problem with the tank. When I pull a pack of mobs with my tank, I generally open up with razor caltrops (PVP assault skill line) before running in and taunt a few of the higher priority mobs. It is more an issue of keeping mobs bunched up so your dps can burn them all down than keeping them all taunted. When I tank, I always kinda chuckle when someone gets agro they can not handle and immodestly their first instinct is to just run away or run to the healer. That is not going to save them, running to the tank will though. I will always taunt the agro off of them and it is easier on the healer if he can concentrate his heals on one area without someone bringing agro to them. I currently have been tanking with a stamina NB (also have tanked with all classes except sorcerer) and although I do not have the use of the chain pull, as long as everyone shows some situational awareness, most trash pulls are not much of a problem. It is not only the tank that needs to prioritize targets, the dps also needs to know what mobs to pick off first in some fights. More often than not when groups fail, the fault is with the dps than it is with the tank or healer.