Diablo III- Skills Article XXVII: The Barbarian, Passive Skills Part Four

Relentless: When you are below 20% life, your skills cost 25% of their usual Fury cost, and you take 50% less damage.

This is probably the best example there is of a skill that punishes the enemy for hurting you.  At 1/4 cost, the sheer amount of hurt you can throw out in the form of Fury-powered attacks is ridiculous, and the damage reduction is not only impressive, it allows you to function in Relentless mode for a startlingly long time- letting you take your time healing yourself back out of it.  That said, this is not a skill to intentionally use- this is a skill to have as an automatic ‘angry button’ so that you don’t have to hit one yourself when your Life gets low (not that you can’t do that anyway and stack the effects....).  Unfortunately, most of the time you’re going to be trying to increase your Life beyond the 20% mark, so don’t rely on having this- just be ready to take massive advantage of it if/when it does trigger.  If you were running around with Bloodthirst, you may want this instead, and possibly instead of Animosity as well.

Brawler: As long as there are three enemies within 8 yards of you, you deal 30% extra damage.

Any Barbarian who’s a fan of area attacks and things like Wrenching Smash or Call of Arreat- or even just who likes to hit a lot of things near them at once- is going to love this skill all to pieces (and break their enemies all to pieces).  While it’s essentially useless when you’re fighting a boss and should be switched out then, it is virtually a given that this will be your third Passive Skill the rest of the time.  Critical Hit loving Barbarians will be hard pressed to choose between this and Boon of Bul-Kathos, ragefest Barbarians will have the same conundrum, tanking Barbarians will have a tough choice between Brawler and Juggernaut- really, just tacking 30% more of every batch of damage you deal onto said batch of damage is patently amazing.  In other words- this skill is just good.  Don’t use it versus bosses, though- it doesn’t mean anything against most of them, and even the ones it can help you against, it doesn’t help you for the majority of the fight.

Juggernaut: The duration of impairing/control effects on you is reduced by 20%.  When a Stun, Fear, Charm, or Immobilize is cast on you, you have a chance of healing 15% of your Life.

Most Barbarians aren’t going to mind control effects too much- with things like Wrenching Smash or Ancient Spear available, you’re pretty much guaranteed to be able to get something near you to hit- or to use something like Seismic Slam to reach out and smack it from a distance.  The exception here is the tank Barbarian, who exists in part to make things easier for everyone else.  A big part of that is being able to rescue allies who are getting jumped by a bunch of enemies, and you just can’t do that quickly enough if you’re imprisoned or stunned for even half a second.  That’s what Juggernaut is for.  Alternatively, it can make a decent third choice of passive for a Barbarian with another angle, though there are generally much more tasty offensive treats for the other Barbarian types.