I agree that some of the changes suggested by Finri are more suited for games with skill trees like those of Diablo 2. But if we stick to adding more variety to weapon expertise, I think Finri has some good ideas. For example, adding a knock down chance to all attacks for 2 handed expertise should be simple to implement since it's already in the game and can simply stack multiplicatively with the knock down chance of whirlwind when the skill is executed. The same can be said for adding a stun/blind chance to unarmed expertise which can add a lot to those role playing as a martial artist.
While we are at it, how about the following bonus effects for the remaining weapon expertise so that no one feels left out
Ranger strength: adds a chance to pierce
Dual wield: damage slightly increases with each successive strike
