Obryn
Hero
IME, it's both.Just curious -- is the main problem in your experience that the Barbarian has low defenses, or that the opponents focus fire on him/her?
I have a high-Dex (ergo high-AC) Barbarian ready to roll, and I'm curious whether she will be dropping as often as you suggest.
Frankly, you will be doing a scary amount of damage every round. As a DM, I will focus-fire the hell out of a Barbarian at every opportunity. Not only will dropping them remove a major offensive target, but their defenses are a tad lower than most other melee PCs'.
Barbarians also have a few powers which open them up to attack. Avalanche Strike is a big one. Every time the Barbarian PC used that, I had just about every enemy on the field gang up on him. It was sometimes even worth violating the Fighter's (though not Shielding Swordmage's) Mark.
-O