What do we mean, when we say that prose is 'beautiful'? When we say that 'beautiful prose' is one of the defining characteristics of literary fiction?
Actually, I dunno. It isn't just that 'beautiful' is dependent on cultural context, or that's it's one of those words devalued by overuse, most often by estate agents. See also 'stunning', 'spacious', 'superb', so on. It's also a descriptor that's both too precise and too inadequate for the kind of prose that rises above the mundane.
We know what bad prose is, because it's mostly, as Toby Litt points out, boring. And we more or less know what genre default prose is, too. (In science fiction, the default used to be Isaac Asimov; now it's Internet Snark.) But what kind of prose is better than default? What kind of prose is, if not beautiful, then truly great?
It isn't the kind of transparent prose that some insist is the sine qua non. The kind of prose that doesn't get in the way of the reader's experience. The kind of prose that doesn't snag her attention. The kind of so-called transparent or windowpane prose that George Orwell didn't actually write about in that essay which wasn't, in any case, about the literary use of language. In any case, how can prose pretend that it doesn't get in the way of the story when it is part of that story?
Great prose is much more than not being bad, or simply utilitarian, or merely competent. It's more than being lyrical, or poetic, or moving, although it can be all of those things. It can be precise and thrilling. It can lure you into its mazes and won't let you go. It can be witty and profound, can work on several registers at once, but it can also be stupidly banal when only stupid banality will do. It's the distinctive voice of the author, or the character, or the voices of both twined in dialogue. Most of all, maybe, as Toby Litt points out, it takes risks. It's a high-wire act. It isn't afraid of failure. It doesn't aim to please. It isn't fan-friendly. It doesn't want to be likeable or relevant. It'll be your friend and bake you a cake and take out the garbage, and it'll seduce your partner and steal your children. It laughs in the face of the grammar police and the impotent artillery of amazon reviews. It hides in your stairway and hangs in your curtain and sleeps in your hat. You know what I mean.