Actually, that might point to a feature that is almost unique to fanfic: protagonists can't deploy the full range of strong munchkin tactics in an original universe without coming off like they've benefited from a deus ex machina, but they can do that in a preestablished universe. Some munchkinry is legal either way, but there are strict limits: you can almost always get away with social, economic, or introspective hacks, for example, because you didn't invent the rules for those. But you can't abuse laws of magic or xenobiology or exotic physics that you invented yourself without threatening suspension of disbelief.

I don't think that works well as a defining feature of rational!fic, though. Almost everything that Bella does in Luminosity would work just as well in a universe where she was, say, a were-badger; the specifics of the setting's inventions don't matter too much.

It's also not strictly limited to fanfiction as such: genre conventions can generate a sort of implied metaphysics that you can then abuse. The Sword of Good works that way, and some of the Discworld books dabble in it too.