Alright, so my primary point is that incapacitated/bleeding (between 0 and -9 hit points) is just too small of a window once you get beyond level 6 or so. Most mobs start hitting hard enough that 10 hit points doesn't make a huge difference, and your chance to end up incapacitated instead of outright dead is a matter of dumb luck. This also reduces the benefit of at least one feat (Diehard) and takes out the fun of being able to save someone on the brink of dying.
Here are my proposed changes:
1. Change the death point to: [Max Hit Points/10] (minimum 10)
2. Change the bleed damage to: [Max Hit Points/100] (rounded up, minimum 1)
So, for a meatier character with 500 hit points, they would be unconscious at 0hp and bleeding at a rate of 5hp/sec between -1 and -49. At -50 hp, they are dead.
So, here are my defenses/arguments for this change.
1. 'Tougher' characters will have a wider margin of becoming incapacitated and being able to be stabilized/brought back that increases in an intuitive way with level. Even squishy characters (say a caster with 200hp) still have an improvement over the current system, although they are more likely to be taken out in a single blow.
2. Diehard becomes useful even at higher levels, as your chance of hitting that bleeding margin increases and becomes a feat that high hp characters will definitely consider.
3. Since the bleed damage increases at roughly the same rate as the margin does, you still get roughly 9 saves between unconscious and dead to stabilize.
4. Any healing at all (even 1 hp) should still stabilize a character. I don't see the additional healing to bring them back to above 0 as a huge burden on healers. They're likely already specced out to bring characters around their level up by 25%, 50%, or even 100% of their hit points.
And one other idea, just to throw out there:
Improved Diehard (feat): prerequisites: Diehard, Increases your bleeding range to 20% of your max hit points.
No concerns on the number of saves there, as having the Diehard feat means you auto-stabilize anyway.