So, one thought is to eliminate them all together. Not a fan of that one.
So, I am playing with this idea.
Criticals occur when the attacker rolls a natural 05 or less on the attack d100. First, this means that the attacker automatically gets max damage for that attack and may pick the area targeted for that attack. This is called a Critical Hit. Since Iridium is an open roll system, the attacker must roll and subtract. Take the amount rolled under and apply it to the Critical tables.
For example, if a 4th level bandit hits with a natural 04, then she must roll again and subtract on d100. If she then roll a 55, she would consult the Critical Table for her weapon at 51 (55 - 4 = 51).
Criticals are rolled on the critical tables by weapon. Punches are considered blunt weapons, while claws are considered edged weapons. If there is ever any doubt, the GM is the final arbiter.
This would make criticals more common but I could rework them to be open ended. I could even put them on one table.
The old way had you rolling a natural 20 on your d20 attack, then a % based on class and level (so a fighter would go up 5% per level). This would mean the chance of a level 1 fighter criticalling would be .25%. In the proposed system your would have a 5% unless I slide the probabilities down shifting the "no results" so we get a longer tail on our curve.