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."