The hardest thing that people seem to struggle with as a Barbarian character is the concept of "simplicity" as related to intellect. In our revised Barbarian packet at NH, we caution new PCs to keep in mind that it is possible (and even common) to be simple, direct and honest without being gullible, foolish and stupid. The national packet includes some interesting (and in my opinion, fun) information about playing the Barbarian superstitions; it suggests that a good Barbarian roleplayer will believe so earnestly in themselves and their rituals and superstitions that non-Barbarians will begin to wonder if or even accept the "simple Barbarian" and their beliefs as wise or factual.
I suppose the most important part of playing a Barbarian is to be confident. Whatever the player does, they ought to stick with it, make it a thing. Who's going to correct the guy (or girl) dressed in a dead animal and waving a sword around?