Threat is at the heart of tanking. However, it's totally hidden from the player, and many players don't understand it well or at all. A thumbnail sketch: every point of damage you do causes 1 threat. Every point of healing you do causes 0.5 threat (unless you're a paladin, in which case it's 0.25 for reasons I've never fully understood). This is base threat; many classes have threat modifiers. Rogues, for instance, have an innate 30% reduction to threat. Warriors and Druids get 30% improved threat in their tanking modes (Defensive Stance/Dire Bear Form); this will be raised to 45% in LK, I believe. Paladins get 90% extra threat for their Holy damage from Righteous Fury.
Then there are various tanking skills which have special threat modifiers. Sunder Armor is one that has been around forever; it wouldn't cause much threat inherently, but it's got extra threat built in. Lacerate and Revenge are other examples.
In short, the way Blizzard has traditionally made tanking work – made sure the tanks are causing more threat than everyone else, so the mobs will attack them – is by keeping their damage low, but raising their threat with threat-increasing auras and threat-boosted abilities. This is not the tack they're taking in LK, based on some trends that have been emerging in blue posts over the last month or two. For instance, Ghostcrawler: "We'd like to get away more from +threat abilities for all classes if we can."
The threat auras are here to stay, but instead of focusing on adding extra threat to various abilities, they seem to just be boosting tank damage. This is pretty brilliant, if you think about it for a second. Tanks have always complained about low damage output, when soloing or for off-tanks that are not needed to tank at some particular instant. Also, threat-enhanced abilities are mysterious: we never see threat numbers (unless you use addons like Omen), and so it can be hard to keep track of what's going on.
But we do see how much damage we're doing, in the combat log or in floating combat text. So why not boost tank damage to (say) 80% of a DPS class's damage? The threat modification auras will boost that 80% damage to 116% threat, again as compared to a DPS class. This will let the tanks stay on top of the threat list, without having to work with too many unnatural +threat abilities. They can just stick to trying to hit things as hard as possible, and rely on their +threat stances/forms/presences/buffs to keep the big ugly guy focused on them. And finally they'll be contributing some serious damage to the raid, and should be able to solo decently. In fact, Prot is currently the highest-DPS Warrior spec in the beta, but that should be considered a bug, and Ghostcrawler has said they'll be rebalancing. Still, expect tank DPS to be very much higher than it currently is.
Finally, this could help simplify itemization. To gain more threat as a tank, just pick up some stats that help DPS (AP, crit, hit, etc.). It's been that way in BC to a certain extent, but the unique mechanics of tanks have given some stats, like Expertise, more weight than they usually have. Of course, many tanking items are still going to be unsuitable for DPS classes due to the presence of mitigation stats like extra armor, defense, shield block, etc., so this won't help with Blizzard's apparent goal to homogenize itemization, but it will mean tanking items, and tanking talents, can help tanks do more damage everywhere.Edit: I would like to clarify that some of the above is speculation and extrapolation based on what Blizzard has said and what I've observed in the beta so far. All I know for sure is that they want to "get away more from +threat abilities," raise tank DPS, and simplify itemization.