Can't XP you, Celebrim, but this is a great post, even though I might quibble a tiny bit with the last sentence. Only because I think most of us choose a system because we already know in our heads how we're going to prepare to play said system, and how we're thinking about how it will be played.
Yes, that's absolutely true. But it's not actually a quibble. It's essential to understanding what I mean by what I said and unpacking its full depth.
You propose GURPS as an example. But, suppose your main interaction with GURPS was through GURPS Supers, and the GM of that game primarily played GURPS from the stand point of a high melodrama game digging into lots of X-Men style soap opera relationships with relatively little dangerous conflict (relying on player optimization Super advantages with their low point cost relative to power to render you virtually immune to most things built without), and then used GURPS potential for deep tactical crunch to really showcase the violent resolution of conflict in an emotional, dramatic, climatic manner. Additionally, suppose the GM dealt with GURPS brutal realism by having an unstated but enforced 'obscure death' rule possibly built through careful construction of setting (either before, or during play), and NPCs in the game sufficiently powerful to intervene in death and resurrect fallen heroes (common tropes in comics, and arguably versimiltude to setting). Instead of building scenarios in detail, this GM tends to create very detailed NPCs and improvs play following player lead - that is, if the players go down to the police station, even if he hadn't prepared for it, there is a clue there of some sort that advances the story. Everything he is doing is 'by the rules'. This is GURPS as a rules light game, digging into the game deeply only when you really need it, rarely rolling the dice, and with a long term commitment to character and story.
Now another player comes from GURPS having primarily played it as a Horror/Survival game. Maybe it was a game centered around surviving a zombie apocalypse, with lots of straight out of a third person shooter tactical crunch, really heavy resource tracking, and leveraging GURPS potential as rules as physics. Character death tends to be high, and the story might actually revolve around an every changing community - a military unit, a village, with no ability to track a single PC long term. The DM also prepares for this game with large highly detailed maps of the surrounding area, and doesn't improvise. This is totally different game using the same rules set. And arguably, GURPS is so silent on how to play GURPS and so sparce on examples of play precisely because it assumes that people will just find ways to play and have fun as long as you provide them mechanics (I don't fully agree, but that's what I think is going on).
Two players from these different backgrounds might be shocked to see the other table in play and there different approaches to creating the game.
I recently was sent by pemerton to read the Burning THACO pdf, and although it didn't achieve what I wanted to see achieved (a module for No Myth play), it was rather interesting in that it largely proved an assertion I had once made, namely, that if you prepared to play by creating a dungeon map, stocking it with monsters and treasures, and then your players came to the table with the expectation that the game was about killing things and taking their stuff, that pretty much every system would or at least could be played like 'conventional D&D'. In fact, very little rules adaptation would be needed I think to BW to really feel like a D&D clone if you set the rest of the ecosystem to model all the non-rules portions of D&D's usual ecosystem. And vica versa. Just off the top of my head, just changing the simultaneous secret declaration to one action at a time (rather than 3 at a time) and doing some things to model Basic D&D's order of play would get you really darn close. The rest, like bookkeeping time and the sort of propositions/outcome cycles expected of D&D, is just something BW doesn't expect to happen and maybe advises against, but which doesn't really forbid and which a pdf like Burning THACO could be seen as inadvertantly blessing because of its ultimate silence on so many important topics.