I run two tables a week, right now we are chapter 4 Horde of the Dragon Queen. Last Saturday with my mature, home group I offered a critical variant they accepted (for now) so take this with all its non-playtested grains of salt. I too, want simple, so this might work for you.
1) Every character has a number of extra 1d6 HD = Con modifier (16 con is a +3 modifier so therefore +3d6 HD, eg). These are NOT used to determine max HP. They are strictly bonus HD (helps with adventuring day as well as crit variant below)
2) Every character has a "wound threshold" (WT) = 1/4 max HP (you can skip this, my players wanted it - ignore WT references below if you ignore this. Or do what I wanted and make "when bloodied" a criteria for crit - see #3)
3) Critical hits Occur on a Nat 20 (use normal Crit rules), when any single attack > WT, when any single attack drops a creature to 0 hp, or when failing a saving throw by rolling a 1 (and the effect does damage).
4) A creature loses 1HD for every one of the above criteria met (cumulative).
5) If a creature suffers a critical and no longer has HD, the creature takes additional damage = 1d6/critical suffered and falls unconscious
Examples:
An Orc rolls a Nat20 against PC. PC takes 2[W]+ str damage and loses 1 HD
A Gnoll rolls a Nat20 against a PC. PC takes 2[W] + str damage that drops him to 0 HP. In addition, the 2[W] + str is > WT of the PC. Therefore the PC loses 3HD
A Kobold does 5 damage to 1st level wizard with 6 HP (6 max) and no HD left. 5 > WT (1.5) so it is a crit and wizard loses 1 HD but doesn't have 1 HD so takes an additional 1d6 damage (rolls a 3) so is now at 0 and therefore takes another crit and loses another HD it doesn't have and takes an additional 1d6 damage...normally this wouldn't matter, but the die is rolledand comes up a 6...poor wizard is dead since the kobold did 5+3+6 = 14 damage to the wizards 6, bringing him to -8 in one hit (PHB pg 76, massive damage)
PS - we haven't changed HD recovery rules yet, but we are thinking a slower HD recovery would feel better (1Hd/day? Con Mod HD/day?). Along those lines, Cure Wounds would gain the Ritual tag and would cure 1HD/spell level used when used as a ritual.
PSS - Nasty crit rules when combined with wraiths and such...use with caution!