I have been thinking of a much more detailed way of doing criticals, and make it a little less random and more appropriate to the creature you are facing.
So first of all you have different classes of creatures, humanoid, quadroped, insectoid, flying and "blob" and different attack types, melee or missile. Each creature has a different area to hit and a percentage chance for each area, for example: Head, Torso, Back,Right Arm, Left Arm, Right leg, left Leg and Wing (for flying). For Large and super large creatures in melee head shots are almost impossible, the majority being torso or legs. Each area has a percentage change of being hit depending on the creature type, for example for Humanoid, head shot would be about 5 percent, torso probably around 20 percent etc. Shields adjust this, making head and leg shots more likely and torso and left arm shots less likely.
A character can "aim" for a certain area to increase the chances of hitting by deducting OB to increase the chances. Each critical then has 5 random roll tables based on the current criticals; Stun/Parry, extra hits, Bleeding, Penalty and Death. So there is a 1-100 table divided up similar to current criticals of increasing amounts for each effect (death would only occur above 80). Different areas of the body hit adjust these rolls. For example, a head shot increase the stun and death roll. A torso increases the Bleeding and extra hits, leg shots increase Bleeding and Penalty etc.
A,B,C,D,E criticals work more like the MERP way, i.e. they mere adjust these rolls up or down (-20 for A, -10 for B, +0 for C, +10 for D, +20 for E).
Combats are great in RM, but these would make them even more interesting!