Much more flattering than my idea that it's just Mark's funky eyes and eyebrows.

Actually, while Luke is the more complex character, I think it's Han that grows more over the arc. Luke is reactive. Events sweep him along, destiny, the Force, etc. Han makes the choice to get tangled up, every time. Luke learns not to be reactive, to take the initiative. But Han just keeps choosing to get himself snarled up: Yavin, Hoth, General, etc.