Hit Points as non-Meat - feats for protecting others

[Segev]

Improved Mounted Combat
You and your mount are as one, protecting and defending each other like a single creature.
Prerequisite: Mounted Combat
Benefit: When you or your mount would take hit point damage, you may choose which of you actually takes the damage.

Intercession
You can interpose yourself between attackers and your nearby allies.
Prerequisites: Dodge, Toughness, or Improved Toughness
Benefit: When a creature adjacent to you or sharing your squares takes hit point damage, you may take that damage instead. Your own hardness and damage reduction and other immunities apply normally to the source of the hit point damage. You may also take on any "rider" effects, such as poison, knockdown, or the like, if you choose. Any effects you do not take on impact the original target as normal.

Look Out!
You are adept not only at turning mortal blows against yourself into glancing ones, but at doing so for others.
Prerequisite: Intercession
Benefit: When a creature adjacent to you or sharing your squares takes hit point damage, you may take that damage instead. Unlike Intercession, resistances, DR, and hardness do not apply to this hit point damage, and you cannot choose to take on rider effects; you cannot physically interpose yourself to take the hit, so only take on hit point damage with this feat.

[Breccia]

Is there any reason why Interecession and Look Out! can be used infinitely and with no restriction on the source?

[Segev]

Is there any reason why Interecession and Look Out! can be used infinitely and with no restriction on the source?

Well, they are limited by your available hp. I'm already a bit concerned they're overcomplicated, restricting on source would get...fiddly. The notion behind hp I'm using here is that they represent whatever reserves of energy, luck, and physical durabilty you have that lets you avoid being KO'd by a blow; these feats let you expend energy and luck (and, in Intercession's case, physical body parts) on others' behalf, keeping them from taking a deathblow (or at least a knock-out blow) even if something does graze them in the process.

So, my counter-question is: why should they be limited as to source or number of uses?

As a further point, one of the major flaws of Mounted Combat is that it's limited to a single use per round, making attacking the mount just a matter of attacking twice to take it out in a lot of cases. So limits like that tend to cripple such abilities to uselessness.

[Breccia]

why should they be limited as to source or number of uses?

Because as it stands, the wizard can stand behind the fighter and he's basically invulnerable to all forms of physical harm. That's way too good for a nonmagic Feat.

Let's try it a different way: the villains consist of one 30 hp wizard and one 100 hp fighter with these abilities. Normally, one of the ways you'd take care of this is to bombard the lower AC, lower hp wizard, including attacks while he's casting a spell to force a Concentration check. By this setup, the fighter can simply take all the damage, the wizard never gets scratched and therefore never fails a Concentration check, because he never has to make one. The two basically pool their hit points, which is a massive bonus to the wizard, who now effectively has 130 hit points less damage done to the fighter (which wasn't his problem before either, and also, the fighter's AC is surely higher so he's taking less damage) and probably the fighter's Fort saves as well (since many Fort saves are damage riders). With "kill the clothie" removed from their options, the party is left hoping they can stop the wizard with what means they have left, such as forcing Will saves (good luck) or trying to grapple. Yes, they had these options before, but "kill the clothie" was typically a stronger option. Even a rogue sneaking up and landing a sneak attack while invisible still doesn't shut the wizard down.

Or this way: the fighter's partner is now a cleric. The two fight side-by-side, the fighter taking all the damage (meaning, again, the cleric has effectively unrestricted casting) until the cleric casts heal (without fear of Concentration checks).

Oh, and let's add this: the fighter has a fire resistance 20. They both take a fireball for 30 and for the sake of argument both fail their saves. The fighter takes 10, then 10 from the wizard, and the fight continues. Without this feat, the fighter takes 10 and the cleric takes 30...or, worse, the wizard takes 30 and hits the ground. That's a huge difference.

Unrestricted damage transference between adjacent allies seems far too strong for a single Feat. 5th Ed Oath of the Crown paladins have a much weaker variant waiting for them at 7th level. Your Feat could potentially be taken by a second-level nomagical fighter.

D&D is like many other games in which combat effectiveness stands at 100% until the target actually dies. As such, two people with 10 hit points each are therefore more vulnerable to two people sharing 20 hp. This is even more so when the attackers are intelligent and know the difference, and would normally focus fire on the more dangerous or weaker opponent.

If this was 5th Ed I would say "make it a Reaction". You can't do that. So I strongly recommend some other kind of restriction, regardless of how you feel about complexity.

Examples include:

1) You can protect one ally per turn, maximum.

2) You can't protect allies from Area Effects that also hit you.

3) You can only protect allies from individual attacks, or targeted attacks that don't target the user.

4) Some die roll is required (such as "make a melee weapon attack DC = the incoming attack roll") so that it can fail.

5) The protector must be aware of the attack before it lands.

6) By virtue of throwing themselves in the way, the protector always fails all saving throws.

7) There is a maximum hp you can lend per attack, or per round, or per day.

8) Since "Reaction" hasn't been invented yet, have it take the effective place: Attack of Opportunity.

9) Also, why is Look Out! worse than Intercession? Look Out! requires Intercession and is strictly weaker.

Sharing hit points is a very strong power. It shouldn't be this easy. Please reconsider.

[Segev]

The 3e equivalent of a reaction is an immediate action. Though I don't care for that, as it's once per round, which is precisely something I'm attempting to avoid.

Look Out! isn't strictly worse; it increases the range to "within reach" vs. Intercession's "adjacent." It is worse in that you can't take the full blow anymore.

In the tactical scenarios presented, I would have somebody try to shove the fighter or the wizard away from each other in some fashion. Given Intercession, the fighter probably tanks those efforts, so in practice, you're trying to shove the fighter.

That said, your points on some limits do make sense and were things I was subconsciously assuming would be needed but didn't put into the rules; I greatly appreciate you pointing them out.

It should require that the character "donating" the hp be aware of the impending damage and free to act. However, I am viewing the "action" being not that much more than the action necessary to expend one's own hp to avoid taking a knockout blow. I could weaken Intercession to the point that, like Look Out!, it only protects against hp; all other effects carry through to the original target. Then, you could use an attack that moves the wizard, and while the fighter might take any damage, it's still the wizard that must resist any effort to move or grapple or the like.