Rational!Harry describes a character similar to the base except persistently Rational, for whatever reason. Rational-Harry describes a Harry which is rational, but it's nonstandard usage and might confuse a few people (Is his name "Rational-Harry"? Do I have to call him that in-universe to differentiate him from Empirical-Harry and Oblate-Spheroiod-Harry?). Rational Harry might just be someone attaching an adjective to Harry to indicate that at the moment, he's rational, or more rational by contrast to Silly Dumbledore.

Anyway, adj!noun is a compound with a well-defined purpose within a fandom: to describe how a character differs from canon. It's an understood notation, and the convention, so everyone uses it to prevent misunderstandings. Outside of fandom things, using it signals casualness and fandom-savviness to those in fandom culture, and those who aren't familiar with fandom culture can understand it and don't notice the in-joke.

I agree that using ! is non-standard outside the fandom cultures. It looked weird to me when I first saw it. Sometimes I am still not sure what goes first, the canon character or the derivative qualifier, especially for crossovers (is it SailorMoon!Harry or Harry!SailorMoon, to take a particularly silly example). However, a special delimiter is needed as a shorthand for "a derivative work based on <canon> with elements of <qualifier>", and space or a dash is not unambiguous enough. The "bang notation" appears to be one of those memetic leaks from subcultures to the mainstream which is likely to survive for some time.

I don't think it's listed explicitly at either of the links, but the principle I'm using is that of hyphenating when you want to make clear that a compound is a compound, and not (e.g.) an adjective happening contingently to modify a noun.

Except Adj-Noun compounds are not actually productive in English. (Also, magnifying glass is arguably from "magnifying" the gerund, not the participle.)