They can self-heal themselves back to full health... they wait and do it when you have them down to about 20% health so if you for some reason can't get a crit in at 20%, you're screwed and have to work them down all over again. And maybe I'm wrong, but it doesn't seem like their armor gets any weaker each time. So really, if you have slow attack speed (warriors, paladins), it seems more like you're fighting a level 38 mob than a level 32 one.

Oh, and don't even think about soloing two of these guys together until you're at least 36 or 37, have AoE, or are in a group... because they'll just heal each other off forever. Sucks.