One minor nitpicky thing...you state in your system:
"To decrease the lethality of this system, you can have the attacker only roll on the Critical Effects chart only if the critical check results in a natural 20, regardless of the threat range of the weapon."
I would say "To decrease the lethality of this system and/or preserve weapon balance, ..."
Also, I think if I used your system I would include some additional effects based on weapon damage type to add flavor. For example, under Light wounds I would add that if the weapon was a puncture weapon the victum bleeds at 1 hp/round until healed or bandaged with a successful heal check. Also, if the weapon is a blunt weapon, the victum is dazed for 1 round. I would rearrange your chart a bit to include critical levels and descriptions in the same area so you don't have to jump around. Something like this:
Light: Bleeding gashes, punctures, or contusions. The victim suffers a -2 injury penalty until healed. Additional effects:
Puncture: victum bleeds 1hp/round until healed or bandaged with a heal skill check.
Slashing: injury penalty increased by an additional -1.
Blunt: victum is dazed for 1 round
Fire: For 2 rounds only, intense pain causes additional -2 injury penalty.
Cold: Numbing effect causes victums initiative modifier to be at -10 for the next 10 minutes.
Leg wound: victum moves at 1/2 speed
Head wound: victum is stunned for 1d3 rounds if a Fortitude save is missed.
Under each 'level' of critical wound you could describe special cases where additional effects occur based on location of the hit and weapon damage type used and all effects that apply would stack.
Really, your system is great as is, and possibly I"m overdoing the game effects of criticals. However, I have found when using the Rolemaster charts that people do like the fact that blunt weapons do more stunning effects and puncture weapons do more bleeding effects, etc.
I'd like to know what other people think...is the increased effects due to damage type something you would want?
