I got this idea from DC Heroes, and I liked it for quite a while. I carried it over to 2nd Edition and continued to use it when I re-started my gaming with my kids a few years back (with 2nd Edition). It served well and certainly made for some excitement, particularly when a second roll resulted in another 20. I thought I had finally figured out how Bard slew Smaug with only one arrow. (I had never really liked the “it must have been an arrow of dragon slaying” theory as Bard claims to have used the arrow previously, which would have expended the enchantment under D&D rules.)
Since joining the retro-clone revolution, however, I’ve decided that I don’t necessarily care for the rule so much. What seemed perfectly fine under the inflated hit point systems of AD&D didn’t look to play so well under the more-restrained Labyrinth Lord, so I simply went with “roll two damage dice” route, meaning a natural 20 on a to-hit roll with a hand axe results in a damage roll of 2d6 plus bonuses. Alas, Bard can no longer slay Smaug in my game, but I think we’ll manage okay anyway.
Under the even-more-restrained system of Swords & Wizardry White Box, I fear that even this system will be a little over-powered and have decided that, inspired by Philotomy’s solution (since aborted), critical hits will deal automatic maximum damage only. This seems more in line with a system where all hit dice are six-sided and the simplicity is great.
Of course, this all applies to both sides of the battle. I’ve given some thought to completely abandoning critical hits, but my son really likes them so I’m hanging with them for now.
Advanced Gaming Theory wrote about Alternatives to Critical Hits Rules and also covered fumbles (which we usually called “critical miss” though that doesn’t really seem to make sense) and I’ll write about my long and winding road with natural 1s real soon.
(Image from Mathisfun)
UPDATE: My son, who I earlier mentioned really likes critical hits, read part of this post and said that he missed the old “if you roll a 20 roll another attack” method. Philotomy wrote about the issue of extra dice rolling slowing things down, and I would generally agree that anything adding complexity to combat needs to be carefully considered lest it bog things down. However, in my experience there is a great deal of excitement surrounding an extra to-hit roll, either anticipation if it a PC scoring the crit or dread if it’s the enemy. This excitement offsets any bogging in my book, so the extra rolling isn’t really an issue in this case.