In Pathfinder First Edition, Diehard was one of my favorite feats. It really felt like a FEAT...my character breaking the rules of the world by remaining conscious and fighting when below 0HP.
Benefit: When your hit point total is below 0, but you are not dead, you automatically stabilize. You do not need to make a Constitution check each round to avoid losing additional hit points. You may choose to act as if you were disabled, rather than dying. You must make this decision as soon as you are reduced to negative hit points (even if it isn’t your turn). If you do not choose to act as if you were disabled, you immediately fall unconscious.
When using this feat, you are staggered. You can take a move action without further injuring yourself, but if you perform any standard action (or any other action deemed as strenuous, including some swift actions, such as casting a quickened spell) you take 1 point of damage after completing the act. If your negative hit points are equal to or greater than your Constitution score, you immediately die.
In contrast, Diehard in the playtest grants, frankly, a boring improvement that doesn't really change things all that much. The character dies at 5 instead of 4 so they can lay unconscious a bit longer or get yo-yo healed one additional time.
Feat: Diehard; Level 1; Common
Traits: General
Skill
It takes more to kill you than most. You die from the dying condition at dying 5, rather than dying 4.
Please consider revising the playtest feat to be more like the old version.
Feat: Diehard; Level 1; Common
Traits: General
SkillYou refuse to go down while there's still life in you. The dying condition no longer causes you to go unconscious, instead you remain conscious but with the slow 1 condition. You die from the dying condition at dying 5, rather than dying 4.
That's a simple addition that, in my opinion, makes the feat far more dynamic and appealing. I don't think it's overpowered by any stretch.