Hit points reflect precisely nothing that is in any way realistic.
I don't care about realism. Or simulationism, for that matter.
Try a different pigeonhole!
Do you have an alternative that's mechanically viable? If you do, please share it. I mean, I have a few, but all of them are more complicated and less believable than the one in place.
I can probably put money on the fact that in June, I will have one that works just fine for my purposes.
As it is, I can still come up with something that solves the problems of the 3e mechanic without taking on the problems of the 'lost ability' that 4e has.
So, let's see, the problems with the 3e mechanic, unless I miss my guess:
#1: Too complex, no one wants to reference the rules in the middle of combat.
#2: Too good. With AoO's, spiked chains, and improved trip, you can shut down almost anything.
My possible solution, using 4e-speak:
#1: Tripping is a standard action melee attack vs. Reflex. A succesfful attempt knocks the victim prone. A failed attempt gives them combat advantage against you on their next turn. The usual common sense rules apply to tripping, like, you can't trip something that is flying, and you can't trip a slug. If it's a corner case, use the Great Equalizer of a -4 penalty on the attack role (grapple, disarm, sunder, etc. all would basically follow this mechanic).
#2: While prone, you give those who threaten you combat advantage. You can fight from prone; the Great Equalizer (-4 penalty) applies to your attack rolls.
#3: You can stand up from prone as a full-round action to avoid provoking opportunity attacks from those who threaten you. You can also "roll away" by shifting. Or get up as a move action and take an OA, you maniac.
#4: Those proficient in tripping weapons add their proficiency bonus to a trip attempt. Improved Trip negates the combat advantage if you fail. Reach works like it does in 4e.
#5: THE RULE OF DIMINISHING RETURNS: Any time you repeat an attack on a target you've already tried to hit with that attack, they gain a +2 bonus to resist it. This is cumulative: if you attack someone 5 times with an attack you've hit them with once, they gain a +10 bonus to resist it. This applies to all attacks everywhere at all times. The reason? Fool me once, shame on me, fool me...you can't get fooled again. (again, all attacks would follow this mechanic). It goes away at the end of the encounter.
#6: THE RULE OF MULTIPLE OA's: Whenever you provoke OA's from multiple creatures for the same action all at once, they get to choose one person to make the OA. Generally, this will be the one that can bone you the hardest.
You want to trip someone? Quick, smooth, not a good idea to try more than once, and not on something that's got good reflexes, but perhaps against something clumsy. Maybe not even a good idea to try even once unless you've got some special training.
You happen to be tripped? Okay, kind of sucks. Roll away, or get up, or eat the -4 penalty and give your enemies some bonii. Might want to do different things depending on how crazy it is. If you think you can take a little damage, get up quick and dish it out.
You want to be good at tripping? Okay, now it's marginally more useful for you in more situations, and you risk less with it. Congrats, you're good at knocking people on their back, you spent some resources, you deserve to be. Of course, it's not your one trick. They can still get up and the more you do it the worse it is for you.
Worst-case scenario: you've been tripped by a rogue and his three rogue friends who surround you on all sides. Prone, they can sneak attack the beejeezus out of you. Let's say you go last in the turn. Oh noes, you are dead, maybe...if they hit you...you still have Mr. AC, after all, and Mr HP, and the sneak attack isn't as bad as it was in 3e. Maybe they hit you, maybe you survive. On your turn, you shift 5' so that only the two hench-rogues threaten you, and use a full-round action to stand up. Now, trippy mc trippsalot has given up his standard action to basically make you move away and spend a round, and he can't do it again quite as effectively unless your party nickname is Stumbles and you have the Reflex save of an elephant on roller skates.
That's pretty off-the-cuff and could use some WotC-style polish, but I'm not gettin' paid to do this.
