The Diehard feat description is confusing.
Benefits:
Automatically stabilize
Limitations:
Disabled
Staggered
First, why both conditions? They are redundant.
Second, what is the point of this feat? Not dying because your Con is low and a healer is unavailable? That's might be useful if there are no opponents around otherwise you get to watch them finish you off.
The name makes it seems like the intention was to allow a "dying" character to heal themselves enough to function but it doesn't work that way. Being "Disabled" prevents standard actions so no Heal checks, drinking a potion, using a wand, scroll, spell-like ability, unless it was swift, immediate, move, or a free action. I can't imagine why anyone would ever take this feat as it is currently written unless they have regeneration (which would still stabilize a character without the feat).
Why not just say "staggered" and "no attacks or acrobatics"?