I've had Sword & Wizardry since at least it went free, and I've had the whitebox variant of it just as long. Quite a while, in other words. And I've even read most of it, even though I don't play with that system, but one thing in Whitebox (and White Star) that I just noticed, and had to re-read a few times to make sure that I wasn't misunderstanding it, was how hit points works. It's really quite different than what I'm used to from the various versions of D&D that I've mostly played. I don't know if it's the way it was done in OD&D or not, because I have little memory of the few times I've played OD&D, but I know that we never used this rule "back in the day" if it still existed in the BD&D, B/X, BECMI or AD&D games that I used to play, and certainly I've never seen it in anything from 3rd edition or beyond.
Every time you gain a new hit die (which isn't necessarily every level, keep in mind) rather than rolling and adding new hit points, you reroll your entire hit point total. If it's higher than it was, you change it, if it's the same or lower, you keep your old total.
Both my AD ASTRA and the S&W:WB systems are designed—or modified, if you will, from the OGL core rule on which they're based in theory—to prevent the runaway hit point problem, which has been with D&D for many years. Looking at a Barbarian from the SRD, for instance, it has a d12 hit die, and gets a new HD every level. In addition to this, it starts with max hit points at first level (i.e., always assume you rolled a 12 on your d12 for 1st level) and it gets a bonus for its Constitution score, which could easily add an addition +3 to even +5 or +6 or so. Every level. A 1st level barbarian, therefore—and for the sake of argument, let's assume a dwarf barbarian who rolled an 18 to Con and added his racial bonus, getting a 20, or +5 bonus, would start at 1st level with 15 hit points. Assuming an average roll of 7 for each new hit die acquired, with a +5 for Con bonus (to keep it simple, I'll assume that ability increases went to Strength throughout this career) would gain an additional 12 hit points each level. At 20th level, his 15 hit points have increased to 243—and again, there are other bonuses he could contract through his career that would increase that even further. And that's average; the maximum hit points he could have if he had really good rolls, would be almost 100 hit points higher still.
This, of course, means that there is no "flatness" to play; a 1st level character and a 20th level character are so fundamentally unlike as to essentially be operating in different genres altogether. (This same analysis can apply to more than just hit points, but for now, let's just keep it simple.)
Characters in AD ASTRA have only 10 levels (instead of 20) and have a specifically designed hit point flatness. Starting at 1st level they have 10 plus their STR score in hit points, and every subsequent level, they gain +2 hit points. There are no HD at all, actually, which means that there is also relatively little variability in their hit point totals. A character built specifically to maximize hit points, for instance—a Carinan hulk with a +3 to STR, who rolled a max STR score to begin with, he'd start with a +7 STR, so 17 hit points. Certainly someone capable of taking a few hits before going down. Using the character backgrounds, he could (potentially) squeeze a few more hit points out of this—if he rolled High Gravity for Homeworld, there would be an additional +1, if he rolled Swept Into Adventure for his childhood, he could potentially get another +1, if he rolled Marooned on an Abandoned Planet for his First Adventure he'd get one more +1 plus an additional +1 hit point every level) and suffered a critical injury for his Critical Event, he could buy cybernetic equipment giving him up to +2 more. He'd have 22 hit points at first level (and a monster STR score, but not anything else.) Assuming he upgraded his cybernetics as soon as he could afford it to squeeze the last +1 out of that that he could, he'd have a maximum hit points at the end of his career at 10th level of 50 hit points.
That's what I consider fairly flat. Keep in mind that most characters, of course, won't be built consciously to maximize hit points, so they'd start with hit point in the low to mid teens, and end up somewhere barely north of 30. In both cases, we get a hit point increase over the course of a career of just over 100%—compared to most likely well over 1,000% for an SRD character. There's a huge, order of magnitude less flatness in the SRD.
A 1st level character is relatively durable; in most cases, he can take a couple of hits in any given combat (maybe more if he's exceptionally durable or the damage rolls are low) before worrying about dying, but even at "high" level, he's still going to be vulnerable (albeit, not as much so) to the same threats that he was at 1st level. A squadron of enemy soldiers; about half a dozen or so with radium carbines and one heavy weapon specialist with a tripod mounted or low slung radium cannon is a serious threat to either a 1st or a 10th level character if he's stupid enough to get ambushed, or to just run up and charge them, either way.
This doesn't mean that I want my games to be "grimdark" with loads of character death—I've purposefully made my characters rather durable, I think—but I don't want them to change genre as they advance. At all points in their career, they'd resemble the swashbuckling characters of your typical Flash Gordon or Star Wars story, and like them, they can be killed in single combat by someone who's either lucky or tactically brilliant enough to put them on the ropes before he attacks, and not because they've had a long boring slog of a combat where hit points were chipped away little by little for many, many turns.
The White Star system, on the other hand—let's have a look at the Mercenary; the class with the most hit points. It advances to 10th level, and gets a new hit die every one of those levels. Figuring the average is mathematically more complicated (so I just looked up where someone else already did the work) but you start the game with 1+1 HD and all HD's are 6. That means that your maximum hit points at 1st level is only 7, and your average is about 4.5 (I'm rounding to the nearest half.) The absolute maximum you'd have, with 10 HD at 10th level is 60 hit points (if you literally rolled 10 6's—not very statistically likely) and your average at 10th level would be about 37.
This is a similar range to my system with three significant differences: 1) you suck at 1st level, and have barely better than a 50/50 chance of dying every single time you're hit. Admittedly, however by 3rd or 4th level, you're comfortably in the teens, and by about 5th or 6th, you've caught up to where an AD ASTRA character would be. You'll actually pass them (slightly) in terms of top range and average at high level, but not by much. Although the range is the same on the top end, the flatness is not. White Star characters have an extremely fragile first few levels that AD ASTRA characters do not, which will tend to significantly discourage swashbuckling heroics at low level. 2) There's a fair bit more variability in the White Star system too—at any given level, you might not gain any hit points at all, and then you might jump quite a few hit points the next time you roll, etc. AD ASTRA hit points are probably too predictable. You always know almost from 1st level, how many hit points you will have at any given level in the future, because other than rolling your STR score at chargen, the rest of it is not variable at all. (Of course, buying cybernetic equipment that raises your STR is the exception here.) Which I'm OK with. Having crocked hit points is really frustrating, and having hit points be a derivative stat rather than a wildly fluctuating (by level) stat that has little relationship to the rest of your abilities isn't necessarily any more logical than the way I do it. Some people just like rolling hit points, though, because it's such a feature of the game and has been since 1974. For them, AD ASTRA's system will probably feel wrong. 3) I did hit points for a mercenary, because that's a lot of hit points compared to most classes. A robot, on the other hand, only has 4 levels and tops out at 4 HD, or an average of 14.6 hit points (theoretical max of 24. Theoretical minimum of 4!) The same is true for yabnabs if you add Companion classes to the mix, although novomachina's reach 11 HD, so they actually do a little better than the mercenary; maximum theoretical hit points of 66, but an average of around 40. Most of the character classes are, of course, comfortably somewhere in between these two points on the spectrum. AD ASTRA doesn't have variable hit points by class like White Star or d20, or any other version of D&D really does. The only variables on how many hit points you have are a) what's your STR score and b) what level are you.
Given that classes in AD ASTRA are more a la carte and less supposedly archetypal, this is completely OK to me. Given that this is supposed to be a swashbuckling action game and levels are not supposed to make you go from a cowering nobody to a superhero who fears nothing, but that you're supposed to be a competent adventurer for the entire game, I think everything about my hit point system is a feature not a bug, and when stacked against this system, I still prefer it. But I can see the point of guys who like to live a little more dangerously and introduce more variability into their hit point totals. Over the course of a 10 level career (it's not necessarily a good assumption to believe that any game I'd run will actually run for 10 levels, though) the law of average will even out the impact of the random rolls considerably, but at any given level, you could be high or low and have to live with it until the next level when you try again.