MerricB said:
...
Consider Rolemaster (and indeed, Star Wars d20), which had Hit Points, but criticals would render that system irrelevant in many, if not most, combats.
Those systems tend to overemphasise the critical hit probability, which is not hard to do with a linear combat mechanic, i.e., a natural 20 always giving a 5% chance to critical, and hard to undo without secondary rolls, which can slow things down.
If you make the probability normally low, and scale it with the difference in skill level between opponents, it can be made to work much better. If scaled with skill level difference, the mighty warrior might be choping through lowly orcs that would slow a lesser fighter down. But if facing an army of orcs, with only a 1% chance to critical each, odds are it won't be a forgone conclusion he lives.
Likewise if you up the critical chance based on situation, surrounded by guard who have you hemmed in with spears and pointing crossbows at you, player may survive but maybe not if a few criticals happen. There's still room for heroic action, but the player's won't necessarily just tally up HP and know they will survive.
Criticals can add another dimension to combat via its different mechanical approach. A "backstab" now doesn't need oodles of multiples of damage to reflect deadliness, instead a vastly increased critical chance can work as well.
Finally, criticals don't need to mean instance death in a single blow, especially for PCs and tough monsters. Although lower than HP, and progressing far, far, slower than HP, the amount of critical damage one can sustain can be set to more than one average blow, but certainly less than 8 sword blows.
Well that's my critical thinking on the subject.
