So...
Shields give -1 to initiative when climbing, jumping or diving
Against unshielded opponents (obviously you need a shield yourself) you get one extra attack before your other attacks. This attack would be at the user's strength with +1 to hit (becuase of a shield's size) but you can't get a critical hit and after checking str against toughness the chart goes as follows:
1 Nothing, your hit goes wide and you miss your other attacks
2 Nothing
3 Opponent may not counterattack, you hit his weapon
4 Opponent knocked back d3", you and he may not counterattack (can knock him off buildings)
5 knocked down
6 stunned (helmets give 4+ save as normal)
That's a bit less OP, since there's now risk involved.