Oops, I was in a hurry and forgot about healing.
Besides just curing injuries, healing magic does 2 things in relation to HP.
First, you can't do the impossible without paying a price for it at some time or another. When you push and fight on despite that wound, you'll bleed more, rip it open, make it worse. When you decide you're going to block all those shots regardless whether a person should be able to do it, you're pushing your body in ways that it wasn't meant to pushed, driving it beyond its normal limits. Healing magic is going to help fix the damage that you do to yourself when you decide that normal human limits don't apply.
Second, the presence of healing magic allows for different rationalizations and new "strategies" (insanities). Powerful healing allows for some completely different images. Maybe the tough warrior really does have 3 arrows in his heart and a shield arm hanging on by a bloody tendon. Since he's going to be healed soon, it doesn't matter that he'll probably die in a few minutes. All he has to do is keep fighting until he's healed or can drink a potion or two. With healing around, people could fight past the point where they should be dead with sheer determination and then get better. It does require some suspension of disbelief ruleswise because, techniquely, they would get better without healing, but in play that will almost never happen between healing spells, wands, potions, etc.
As an alternate idea that's part of point 2, having healing around might change the way people do some things. It might be acceptable to take a few hits to maintain a position or stance where the fighter can drive home a truly leathal strike, instead losing that positional advantage because you had to dodge back, or turn your attack into a parry.
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I guess everyone wants to discuss HP because they're one of the most controversial parts of DnD. Over 10 levels, an increase in pure wound taking ability of 6 to 10 fold is a bit hard to rationalize, and thus often criticized.
While in many cases, your point about the same # of average hits needed to drop a target is correct, I do see some change in the number of hits. It will often be 5-6 instead of 4-5, for example. However, that could be included in your idea by simply adding a bit more HP as well as defense.
However, spells bypass DR, so stuff like magic missile would still be incredibly dangerous. Truestrike would be safe, except in the hands of Sor 1 fighting class X which has a decent attack already, and then throws truestrike and few points of Power Attack to beat DR. It's already a good combo, and it would be even more brutal since truestrike could easily beat the class defense bonus at mid levels and the character could already have a high STR attack good enough to beat through DR. Magic Missile doesn't give crap about DR, so it would start cutting through people.
I suppose you could change DR so it does work on spells, but that would really screw with alot of things.
However, a big problem would be that the chance to hit bottoms out at 5%. After that point, additional increases in defense would be meaningless. Dodgy type characters might start putting on armor to get DR since their defense is already maxed.
Also, missing a lot is quite frustrating. I get very annoyed when fighting monsters like incorporeal undead because their main defense makes people miss all the time, regardless of strategy.
Finally, finding the proper damage scaling point would be hard. Fighters can do quite a bit of damage. So can wizards. However, most of a wizard's attack don't care about Defense, and do care about Reflex and HP and SR. A fighter's attacks care about DR, AC, and HP. If you make unbypassble DR in the form of armor commonly availible while you increase AC, fighters are in bad shape. Even if you make armor DR work against spells, wizards usually drop damage in bigger chunks (10d6 once instead of 1d8 +10 twice), so they still aren't hurt to badly on their big guns. You'd have to chance all the damage dealing spells around, and might throw another imbalance in. For example, many spells use Ray attacks. It's assumed that these attacks will be pretty easy in most cases (with proper targeting, ie, you still can't hit the monk). Adding a big dodge bonus to AC will screw with the way these spells are supposed to work.
It seems like a lot of complicated changes for a relatively minor effect.
However, it would provide a nice, indirect nerf to Harm.
