So it seems obvious, but given recent conversations I want to reiterate something:
A game is *about* what it gives *mechanical attention* to.

(a thread now ensues)

The mechanics presented shape play. Things the mechanics care about will become the things that matter when the game is played.
I should note that I say 'that the mechanics care about' rather than 'that the mechanics are for'.

For example:
A game with a robust system for starvation & thirst, the weather, sickness etc clearly cares about wilderness survival, even if the specific tasks of wilderness survival don't get much mechanical detail.

a related detour: I frequently see the claim that 'poker has no mechanics for bluffing' as a counter-argument to the whole System Matters ethos. And this is a bullshit argument.

Poker has hidden information, variable states which can give one player a (hidden) advantage over another, the ability to fold in order to mitigate loss, a system that allows you to signal confidence by how much you invest in a gamble.

The game is fine-tuned to produce bluffing as an emergent behaviour, even if it doesn't have mechanics that tell you how to bluff. The rest of the game points towards bluffing as an expected mode of play.

It's the same with RPGs. Think about the tools players get to affect events in the fiction, and how hard or soft those tools are relative to each other.
(for what I mean by hard/soft tools, here's a little blogpost: https://cavegirlgames.blogspot.com/2020/11/theory-hard-soft-tools.html )

Likewise think about where the game lets you mechanically diferentiate your character. What things the game gives detail to, feels it needs to catalogue extensively rather than abstracting away. The terms by which things are defined in the game.

This isn't to say that the fiction and presentation of a game don't matter - they absolutely do! But if that fiction cares about one thing, and the mechanics care about a different thing, then the game you end up playing will probably not be what you initially signed up for.

This is a problem with, say, VtM. The fiction positions itself as being about morally reckoning with the demands of existing as a vampire: the need to feed on people, & the corrupt, power-hungry nature of vampire society. It asks how much PCs are willing to compromise to survive.

But the mechanics? Those are mostly (at least prior to v5) about doing violence and having super-powers. So the way most vamp games tended to go *in practice* isn't what the fiction advertises.

Good design, then, has the fiction and tone line up with the mechanical elements. Thirsty Sword Lesbians, say, wants to be a game where intense emotions are revealed by dangerous high-stakes situations.

And the mechanics support this. There is *literally* a move for when you finally kiss in a dangerous situation! And it makes the kissers better at supporting each other! Where TSL wants something to matter in the fiction, it gives it mechanical weight.

Two counter-arguments I see, and disagree with.
The first I have some respect for, the second I don't.

firstly "you can just hack the game to be different!"
This is true. But when you hack a game, you're not playing the game as presented. You're designing your own game that iterates on the original.
That you alter mechanics to get the result you want is a demonstration of my point

second "you don't have to use those rules at all! you can ignore that mechanical weight!"
If you're doing that, what is the fucking point of playing that specific game instead of just going freeform? If you're ignoring the rules, why did you bother buying the game to begin with?

If you sat down with a game about romance with romance-focussed rules, and then ignore all that to run a dungeon-crawl, then you are not engaging with the game before you. Your failure to engage with the game doesn't mean it's *not about* romance, it means you're just ignoring it

In conclusion: mechanical weight and focus determines what a game is about in real practical terms, unless you aren't bothering to actually engage with the game. Claims otherwise mostly come from ignorance, sometimes from wilful perversity.

"System Matters" is, to me, a statement so blatantly obvious that it borders on tautological. And yet, here we are explaining this stuff yet again, and that baffles and depresses me.