There's been a lot of debate over how, exactly, taunt works. Some warriors swear by "spam taunting", some say it's a matter of timing. I've made it through 65 levels as a warrior, so here's my experience with the matter:
1...No, taunt does NOT slow down your attack. It's a completely independent timer.
2...Assuming taunt skill is maxed, it seems most effective to wait and use it as you lose aggro rather than spamming it during the battle. If it does indeed work by putting you just slightly ahead of the person with the highest aggro (when it succeeds, then spamming it will do nothing when you already have aggro, expect guarantee that you cannot taunt again for 6 seconds. Say your caster nukes right after you have taunted - he gains instant aggro, and all you can do for 6 seconds is beat on the mob and hope you can gain aggro back. Meanwhile, mob is beating on caster, possibly reducing caster's HP to the point where the caster has low-hp aggro, and now there's NOTHING you can do to get aggro back except wait for the caster to die. OR...caster is running around like a madman trying to keep the mob from beating on him, preventing you from beating on mob and possibly preventing you from getting in range to get taunt in when it DOES pop...and possibly running into other wandering mobs and getting aggro from them too. On the other hand, if you stop taunting while you have aggro, and caster nukes, the mob turns away from you, you instantly hit taunt, you get the mob's attention again (hopefully) and the fight continues.
3...if you want that continuous growth in aggro, make sure that you get the provoke line of abilities (at least I think it's provoke....it's Incite at lvl 65). Each use of this ability adds a set amount of aggro, with an "aggro-over-time" component. It's usable every 30 seconds, is great for peeling a mob off the puller or slower at the beginning of the fight, and as far as I can tell, ALMOST irresistable.
Now, of course, all this applies mainly to being a warrior. If you are a knight or a ranger (rangers tanking....::shudder::... but it happens...) then you have a wide variety of spells to choose from that can help you lock in aggro much more reliably than taunt. Stuns, snares, and roots all generate quite a bit of aggro, so if you're a hybrid tank, you'll want to keep one of these handy at all times.