Some people just want caster supremacy as a preference.
I'm likely beating my head against a wall here because I know for a fact that this has been mentioned over and over and over again, but what the heck? One more time?
Having casters be the only ones who do reality bending things =/= caster supremacy. Once again, you completely ignore all of those mitigating things that are very real and important factors. Things like being extremely fragile in combat, spell limitations per day, the chance of even finding certain spells, finding components, concentration/interruption, the list goes on and on and on. In actual game play, for decades of D&D since the beginning, casters were not supreme unless you made them that way or advanced to 18th level in AD&D. Which, by the way, hardly anyone did. For 99% of gameplay, casters were not the end all/be all supreme class.
These things are
important, and the continued refusal to acknowledge them as such by you and others is extremely disingenuous.
For that to happen, non-casters, or martials, have to be less complex/capable/interesting than casters.
Wrong again! Maybe less complex, sure. I'll give you that one, but that's by design because an awful lot of gamers don't want complex. How hard is that for you guys to grasp? As far as capable and interesting, that's already been debunked over and over and over again. If the stars align you can have an automatic success once or a couple of times, or you can have a good chance of success but do it as much as you want =/= objectively more capable because it ignores the context of the game. And as far as interesting goes, that's entirely subjective.
This is why they are arguing for this. Arguments about "dissociative mechanics" in a game where two of the most universal aspects of the game are hit points (that fully replenish after sleepy time!), and armour class (where armour makes you harder to hit) end up being laughable (no offense). When people point out that the fighter can do things, but ignore that no other class is worse than them (in terms of how they are mechanically put together as a class) at the exploration and social aspects of the game, that's disingenuous.
What's disingenuous is the fact that you just totally ignored the statements even from just today about how HP aren't disassociated, but abstract (two different things), and how AC is also abstract as a way to reflect the abstract combat round. All abstract. NOT disassociated. And you also ignored the several times where it was brought up that every other class is not "no worse" than the fighter, because the fighter gets two
extra feats above and beyond everyone else, which clearly boosts them well above every other class.
I know you've read these comments, so why do you choose to keep ignoring them?
People have given many answer's to the OP's question, but the worldviews are too different; some people like balance across classes for a few reasons ( 1. So no player feels his choice of class will disadvantage him compared to others; 2. To mimic certain fantasy genres; 3. Etc.) and some prefer caster supremacy because magic should out-do everything in that worldview. I honestly don't get a lot of aspects of the latter worldview: When people choose to play fighters in games of that worldview, why do they choose such an inferior class? If magic is so superior, why aren't more people in the game-world wizards? There's more, but at the end of the day, it's just a preference. And that's fine.
What isn't fine, is people demanding that their worldview on magic and martial areas of the game necessarily exclude all others. What I have come to understand from this thread, though, is that the answer to the original question is "nothing"...if you come from a caster-supremacy worldview.
This is one big pile of a steaming strawman. No wonder you can't figure it out, because the argument in your head is one no one is making. This whole thing is based on the premise that we're acknowledging that he fighter is inferior, or that having a class that is mundane is always inferior to a spell caster. Neither is a true statement.
No, the reason we keep having threads like this is because they always go like this:
"The fighter without superhuman abilities/powers is an optionless beatstick."
"No it's not. Here's a list of all the things the fighter can do, and here's how you do it."
"The fighter without superhuman abilities/powers is inferior to casters."
"No it's not. Here's a list of all the ways in which they are pretty equal, between the fighter's abilities and the mitigations of casters."
"The fighter without superhuman abilities/powers is an optionless beatstick."
"Wait..didn't I just answer that?"
"The fighter without superhuman abilities/powers is inferior to casters."
"Ok, you know what? Nevermind."