Originally Posted by
Slipjig
While I agree it FEELS wrong for a Barb to be vulnerable to fear, it's important to remember that Fear effects generally aren't a "This is a scary monster, so I'm Frightened" thing. It's generally a "there's a magical effect attempting to trigger a specific emotional response" thing. The level 17 Barb isn't actually afraid of the Quasit, the Quasit is triggering a magical fear response. And if "weak against mind-effecting magic" is part of your class, it makes sense that even the low-level versions might still be able to effect you, and you have almost no chance (without external support) against a boss monster that target that weakness.
Levelling up doesn't always mean we outgrowing our weaknesses. We still expect an ice elemental to be vulnerable to fire, no matter how many hit dice it has.