Fun little topic, I hope. What do we think the importance of the chunky eyebrows could be? There's a couple of references in enemy taunts and so on to them perhaps being unusual in some manner, and I thought it'd be fun to speculate. I reckon they allow the adventurer to understand their enemy, i.e. to translate what the enemy is saying.
I think they confuse and intimidate your enemies, resulting in +10 for you and -10 for them. But since it's always on the game doesn't display it. You could imagine, however, how much more difficult it would be for an adventurer without big eyebrows.
I always presumed it was to do with spell casting, and its a facial expression that grants them and due to the exceptionally bushy nature of the hero that they are able to cast strong spells.