The mobs are hard coded to try their best to surround you. In inferno, this is almost inevitable death. Watch the movements of the mobs, try to pre-empt them, and always keep an escape route in mind should things show signs of becoming overwhelming. I'd recommend you always keep your leap skill at hand; remember it works defensively too, allowing you to jump quickly out of a group that's surrounded you, or even to jump out of the enemies' reach to another level of terrain.
For normal and most of nightmare, focus on the skills and equipment that allow you to do as much damage as possible. On these difficulties the enemies don't do nearly enough damage to overpower you, especially when playing in a group, and just having armour is enough. You'll need to balance things a little more by Hell and Inferno however, or the barbarian's tendency to get up close and personal can get you into a lot of trouble very quickly.
some more: http://www.squidoo.com/diablo-3-barbarian2