Scourger: What someone was pointing out was, if you keep the normal save mechanic (natural 20 auto-success, natural 1 auto-fail) and 20 1st-level commoners swim across, say, a 10-foot pit of lava, 1 is likely to hop out on the other side unscathed. Not getting splashed by a bit of lava. Swimming in it. Modern scientists can't even approach fresh lava without a special heat-resistant suit, and can't stay near it overly long; actually diving in would kill them regardless. Likewise, if 20 villagers each throw a rock at a passing dragon, 1 is likely to kill the dragon. How? Hit him in the eye and send him careening down to the ground and landing on an upraised pitchfork going through his other eye and into his brain? Because that's about the only (ridiculously) feasible way. An unlucky PC could die suddenly from just stepping on a caltrop.
And how will you differentiate between normal and subdual damage? Some weapons just aren't capable of killing someone unless striking very, very hard at a specific weakspot, and should only ever knock someone out or bruise them. Maybe subdual attacks would stagger a character on a failed massive damage save, then knock them unconscious on a second such failed save (or something along those lines, anyway).
So, basically, what I'm saying is that you should say the natural-1 and natural-20 rules don't apply to these massive damage saves. You should also provide at least some mechanic for being wounded but not dying. I.E. failing a massive damage save by less than 5 points might just leave the character wounded, and perhaps three wounds would send a character to -1 HP, with two wounds disabling them (0 HP), and one wound either being negligible or perhaps partially disabling (i.e. restricted to a partial action each round but not in danger of dropping to -1 from strenuous actions; or perhaps a -2 circumstance penalty on some stuff, like attack rolls, AC, checks, and saves).
BTW, if you do use some sort of wounding, unconsciousness, or staggering effects, you might have to specify how much natural or magical healing will negate them.