Two things to mention here. Nothing particularly controversial… the first is a qualification, but the other is a house rule that modifies the existing Pathfinder rules for Injury and Death. If you’re not sure what the official rules are then they start on p189 of the Core Rules (2009) or you can take a gander at the Pathfinder PRD instead.
In the rules as written if your character takes either 50 points of damage or half your maximum hit points (whichever is higher) in one blow then, assuming you aren’t already dead, you are judged to have taken Massive Damage. This means you must succeed at a DC 15 Fortitude saving throw. If you fail, then you die. The reason I’m mentioning this is because we’ve only ever used this rule sporadically in the past. I’d just like to underline that I want to fully incorporate it into the game and use it all the time.
I was toying with modifying it. There aren’t any further means within the Pathfinder rules to do it, but the old third edition Unearthed Arcana (2004) had some sensible sounding rules for alternative Massive Damage Thresholds. They basically increase the damage threshold depending on how large your character is. However, your total hit points is already a very good way of doing this… and by saying that the massive damage threshold is either 50 or half your maximum hit points, I think Pathfinder has its bases covered pretty well.
In the official rules, a character starts dying when the are reduced to negative hit points. They remain in the ‘Dying’ state and can be easily revived as long as they hit points doesn’t fall to a number equal to or greater than their negative Constitution score. This replaces the flat -10 hit point rule that existed in third edition, and in fact we’ve been using the ‘Minus Con’ rule in the game since long before Pathfinder was even thought of. It’s a pretty obvious rule.
However, what I’ve noticed while GMing is that ‘Minus Con’ isn’t always a large enough negative hit-point buffer. When your character reaches mid to high levels he dreads being almost killed by a blow, as it’s obvious that the next blow will undoubtedly kill him outright. Therefore I instigated a new rule that said this negative hit point buffer increases as you gain levels. I’ve never used as generous a buffer as 4e, but I think it’s been helpful in keeping the game on track and avoiding unnecessary PC deaths.
The rule is that characters die when their hit points are reduced to a negative number equal to or greater than their Constitution score, or 25% of their total hit points, whichever is greater. So a 20th level fighter with 200 hit points wouldn’t die until he was reduced to -50 hit points. That’s far more in balance with the threats he would be facing and the damage he would be taking at his experience level.
D&D Next have a different approach. Their negative hit point buffer is a negative number equal to the character’s Con Score + their character level. Is that a better way of doing it? Not quite as fiddlesome as having to work out 25% of your total hit points. Also not quite as powerful.