There's been discussion since pretty much forever about critical strikes in Rolemaster, often focusing on how lethal they are. For fantasy, so long as you have magical healing close at hand, I tend to think they strike a reasonable balance. But if you're into a more methodical approach to the idea of crits, there are some interesting points.
I don't know how the A-E idea got started for crit severity, but it does bear a striking resemblance to the Abbreviated Injury Score-Code system developed to measure traumatic injuries (it has six ratings, although only five are used for injuries that are not essentially instantly fatal - total severance of aorta is an example given for AIS-Code 6). Of note, the AIS-Code table lists both examples of the type of injury considered to fall in each code AND the probability of death from an injury in that code.
The AIS-Code version of an A Crit would likely be 1 or Minor. For that level of injury, there's at most a 1% chance of it being fatal. Reasonably close to the one fatal result in the A Krush table in RMU (which works out to a 5% chance based just on total number of results, not the roll spread). Interestingly, the same table has five fatal results in both D and E (25% based on results, not roll spread), while the AIS has a 16%-30% fatality chance in Code 4, and a 30%-99% in Code 5. The C column has 3 fatal results (15%), on the high end of the 2%-16% possible fatal rate from the AIS system. The B column is even more lethal (10%), with two fatal crits as compared to a 1%-2% chance in AIS.
Going by that system, both A and B crits seem to be more lethal than might be expected using AIS, and E if anything is rather understated by comparison. C and D both run fairly close to the real-world model.
Just throwing this out there because. I came across the information while working on rules of my own and thought others might be interested as well.