A wonderful Warrior Aggro thread on Blizz forums

[Darian]

Why isn't Taunt on that list?
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Taunt (and its cousins Challenging Shout and Mocking Blow) don't really add hate. It's possible that they do have a hate component; the only way you could test that would be to have a warrior use each of those three skills in the absence of any hate other than proximity, and have someone else perform one action which generates the minimum amount of hate. If the warrior retains aggro after X seconds, then the ability does carry hate with it. (Mocking Blow is a small exception, as it does carry some level of hate generation since it does damage in addition to forcing the mob's attention.)

The real effect, though, is that rather than adding a set amount of hate in order to put the warrior at the top of the hate list, they directly but temporarily move the warrior to the top of the hate list for X seconds. If the warrior's not doing anything else to generate hate, at the end of those X seconds, s/he'll move right back down to where s/he was before using the ability.