Diff'rent Strokes: Barbarians and Varying Mechanics by Class

[ZombieRoboNinja]

I think the cleanest mechanic for rage would be to give you a big damage boost, an automatic 20 on all Strength checks, and the ability to keep fighting well into negative hit points; but you take some damage when you come out of it.

This puts limits on raging, without requiring the player to track an additional resource, and it lines up very well with the fiction. You can perform feats of inhuman prowess, but you're causing yourself stress and injury in the process, and you'll feel it--maybe even die from it, John Henry-style--when the battle is over.

This would be a quite good option for a berserker theme, I think, but it might not be enough to define a whole class. I'd actually like to see both as possibilities.