My Two cents :
The Barbarian will be a controller.
A defender must be 'sticky' ; he must be able to hold a front. The 1st level dwarf fighter Kathra can do that : She has a threat range (as opposed to all five other PC), stops the movement of anyone she hits with AoO, she can mark a foe that would have -2 to his roll if he didn't attack her, and if someone tries to shift around her, she can push him back with her shield in order to control the battlefield. She looks like a great defender. The paladin doesn't look as good at this job though his marking ability is nastier ; unless bad guys love taking free 1D8 of damage, the Paladin will get the attention of anyone he challenges.
Allong with good AC and HP, these are what really makes a defender : People can't easily ignore them and rush the more fragile PC, they have to get them out of the way first.
I don't see the Barbarian having these kind of qualities and he probably won't be able to efficiently hold a front. In fact, it goes against his main power : Rage. You can't go berserk and then patiently hold the line.
A controller is supposed to be able to take on a large number of foes at the same time. Well, if the barbarian is very mobile and if he has great area effects (Whirlwind style of attacks, fear effects caused by his rage etc.) he could do just that. It would fly in the face of the traditional 'Artillery' method, but it would play the same role.
Of course, if your controller is personnally jumping into the fray, the defender is free to work more closely with the striker (In that configuration, I'd want a Warlock as striker) and not be as worried about protecting a weak mage.
Just a thought.