I'm a picky bitch about my RPGing, striving for a crunchy system that strikes the best blend between verisimilitude and usability. D20 in particular has always bugged me for a few of my opinions:
Ablative HP per level as a health mechanic sucks. Higher level characters shouldn't be able to roll around naked in a minefield just because the mines only do 1d10 damage apiece. A person's health, in the sense of incoming damage they can absorb, shouldn't be constantly increasing for no good reason.
Levels kind of suck. A corollary of the above, character growth being paired entirely on a concept of levels leads to things like higher-level people being able to ignore being constantly stabbed. I'm also not a fan of long periods of zero advancement followed by a sudden leap in power when you ding the next level.
Skills as a pass/fail single roll aren't interesting... D20 has fairly lavish attention to spellcasting and some interesting mechanics for combat, but almost no complexity whatsoever for non-combat skills. Doing surgery? Roll 15+ on a d20. Underwater lockpicking? 15+ on a d20.
...but Powered by the Apocalypse blows. You might be tempted to think, "Oh! That new-fangled PbtA system doesn't use a pass/fail skill check..." Let me stop you right there. Powered by the Apocalypse is a sham. The way it's set up, roll 2d6 and the most likely result is that you semi-succeed, is intentionally aiming to get every action a player takes into a bargaining scenario with the GM. 2d6 is so swingy that even if you have a specialized character with every conceivable bonus, you're still pretty likely to halfway botch the thing you're best at, which ruins the world for me. Since you have to have a total of 10 or better to uncompromisingly succeed at something, even if you have a +3 bonus, which is a very large bonus, you have to get a 7 or better, leaving a 41.66% chance of partially failing. That is fucking terrible. And it's terrible in service of the belief that having to go back and forth with the GM to negotiate the result of every roll is making the game better somehow. It's not.
Weapon damage being static is bad. A weapon that can only ever do a fixed amount of damage lead to scenarios where a given weapon cannot possibly defeat a given opponent, even taking a critical hit into account, barring houserule stuff like three nat20s being an instakill. You can have the best swordsman in the world and he won't ever do better than 2d8+8 damage with a 1d8 longsword and a +4 strength bonus, no matter how amazingly he strikes.
What systems do I like better?
Alternity A colossal failure that crashed and burned, but I think it did a whole lot right. Your health pool is based off of your stats, not your level. It does have classes and levels, but those serve more as a framework for what skills and feats one can buy than anything else. A character's skill ranks have much more bearing on what they're doing than their class level. Plus the skill checks aren't just pass/fail, there are different degrees of success and failure depending on the roll. And as a side-benefit, weapon damage scales on how good of a result you get on your attack; you can't ever ignore a random person with a dagger, because getting a dagger stuck somewhere sensitive is bad.
Shadowrun Since I'm not a fan of level-based systems, it shouldn't be a shock that I like skill-based systems. Shadowrun has its own issues of course, but I approve of its progression where you can purchase character upgrades piecemeal rather than the all-or-nothing of playing multiple sessions with no advancement, then getting lots of new stuff because of a level increase. Also has successes on attacks directly feed into a weapon's damage, so like Alternity the better your skill the more likely you'll take down a target.
Fantasy Flight Star Wars/Genesys I'm not big on custom dice, but that aside there're a lot of good things here. The system is geared for murky resolutions of imperfect successes and failures with upsides, which is far more interesting than pass/fail binary results. And the important distinction between Genesys and PbtA is that the "you succeed but there's a downside" is not a foregone conclusion with every roll, but a result of situational disadvantage dice added to the pool for challenging conditions, and can be negated by advantage dice for having good tools/people helping/etc. If you partially fail it's for a specific reason and not because every roll has a high chance of it. Advancement is through purchases of skills and talents in bite-sized XP spends, so you can usually upgrade your character with every session. Character health can be upgraded but not to such a degree that they can disregard incoming bullets, and the system does a good job at capturing a cinematic adventure feel by having disposable minion NPCs, decent challenge rival NPCs, and downright scary main villain nemesis NPCs, and it does so without any of them being HP batteries.
(New) World of Darkness Old WoD was clunky as hell, a single combat would take all night with roll to hit, roll to dodge, roll to damage, roll to soak, the nWoD update helps hugely with streamlining things. I prefer the oWoD settings, but it's pretty undeniable that the newer ruleset is a big upgrade. Like the other systems, it's more skill-based than level-based in progression, attack damage scales with attack roll successes, etc.
2nd edition Warhammer Fantasy Roleplaying/40K The percentile roll is refreshingly easy as a mechanic, while still having a degrees of success mechanic instead of just pass/fail. I'm also a big fan of the advancement system where characters can easily purchase upgrades within a career and change careers at will, it's considerably more organic than locking into a class and gaining levels. The over the top, gory critical hit tables are also a guilty pleasure. The system is geared heavily towards the setting, I don't know how well it would adapt to a non-Warhammer game, but it does a great job of being a gritty and uncompromising ruleset for a dour setting.
So now that I've gone on at exhausting length about my opinions on rule mechanics, can anyone recommend a system that I have yet to mention that could also fit the bill? I'm always on the lookout for interesting ways to play.