Question about Taunt and Taunt auras

I'm not a numbers cruncher, so I can't show you exact numbers.

Tankers have an inherent power called Gauntlet that has a +agro built into it.

Tanker attacks and damage/effect PBAoE's have a base +agro added to them from the individual power (in most cases) and the additional +agro from Gauntlet.
If another player can do enough damage, they will pull the agro away from you.

Taunt (the power) is by far the strongest and most powerful +agro tool.
If you have an enemy Taunted (that is with the Taunt power), the only one that will be able to pull the agro away is another Tank.

For this to make sense, here's basically what went on.
Originally, there were no inherents and Tanker attacks didn't draw any more agro than other attacks did.
The only way for a Tank to hold agro was Taunt.

Many Tanks were build that didn't have Taunt.
The DEVs look at the AT and think "the Tanks aren't taking Taunt and that is really what they are supposed to be using to defend their teammates. That, after all, is what Tanks are supposed to do" (this is a mythical guess about actual dialog)
So over the course of time, Tanks were given the inherent Gauntlet and also had +agro added to their attack powers - so that they would be pulling agro even when they weren't intentionally trying to do so.

These powers aren't a strong as Taunt, but they at least make a Tanker do at least some of what they were intended to do - draw agro in order to protect the team.
So if you are a Tanker that really wants to protect the rest of the team, take Taunt and use it.

Taunt won't get everyone in a multiple mob situation, so you will need to slot some recharges and click through targets to get different parts of a large mob each time if you want to keep all the agro.

But this is all from someone that jumps char/servers/sides/ATs like crazy, so I do have experience with Tanks, but I am not a "full time Tanker", so maybe that actually makes my advice better in some cases as I am actually seeing how Tank behavior affects the rest of a team.

The way taunt works is the same for all powers. Durations don't work differently in an aura than they do in powers. That said, when you see weird numbers in game (like 400% taunt) that's a bug with how Real Numbers is displaying them - that's the taunt's MAG (which is largely irrelevant except in some extreme corner cases). If you want to see the info, the best place to go would be City of Data (as New Dawn linked more directly to below).

Circeus's guide (highlighted) was written long before Castle (with the help of Ghost Widow) told us how Taunt really works. Some of the info is right, some of it completely wrong (most notably 17 and 26). I wouldn't trust it anymore.

[edit: I say that with no disrespect meant to Circeus, btw.]