Lanefan
Victoria Rules
Unless there's elements (traps) or items (sharpness/vorpal weapons) or effects in the game that force it to occur, at which point you do need mechanics: not so much for decapitation, as the results of that are usually both quick and obvious; but for limb loss and so forth. Sadly, no version of D&D has ever really addressed this; I (and probably many others) made up my own rules for it ages ago.Decapitation in games is a little bit more acceptable, but not by much. If I feel like anything I do has a chance of invalidating all of my previous work, then I'm unlikely to want to do anything. If I'm swinging an axe around, the chance of me cutting my own head off is significantly less than one percent. Even if someone is swinging an axe at me, as long as I'm wearing armor, the chance of decapitation is not high. Decapitation in combat is one of those unrealistic things, that happens in movies for dramatic effect. Basically, if you have a game mechanic to address such a thing, then you've already vastly over-estimated how often it should occur.
I absolutely disagree. For me, trying to shoehorn everything into core mechanics is poor design which inevitably leads to elements in the game not working as elegantly or smoothly as they otherwise could.From a game design standpoint, incidental mechanics which bypass core mechanics are a bad thing.
Core mechanics should be nothing more than the default used when there's no other mechanic in place.
Again, disagree. I'd rather see a system where sometimes - not always, but sometimes - having 100 h.p. as a fighter leaves you exactly as vulnerable as the magic-user who only has 25.Critical hits which bypass armor are a bad design. Random decapitation which bypasses HP are a bad design. If you want an axe to have the possibility of killing someone outright, then you should work with the existing core mechanics, and make sure that the high end of the damage roll is sufficient to kill anyone you think should be vulnerable to insta-death.
Your idea about the high end of the damage roll for the axe being enough to kill someone doesn't address this at all: if the damage roll is, say, 2d50 then it's almost never going to kill the fighter but will nearly always kill the MU. But if the axe does the usual d8 damage but has a small chance of removing a limb or head on any hit the vulnerability factor gets much more even. The vulnerability is even more balanced when the effect of a trap or whatever is to remove a limb or head without regard to h.p. at all.
Otherwise having lots and lots of hit points just becomes too important.