I've been reading a thread on a forum about what makes 'good' prose. Needless to say, the camps were quickly established: 'fancy, pretentious tosh' versus 'banal, lowest-common-denominator crap' versus... No, I won't go on, you've heard it all a hundred times. So instead I've come over here to sort out what I think and, as so often, what I think is: it depends what you mean by 'good'.
The basic level of 'good' prose, it seems to me, is 'functional'. It does the job for the book it's making: conveys the story and characters adequately, doesn't baffle the reader, keeps them reading to the end. And then there's 'good' as in a bit more interesting than that, conveying things more than adequately, getting the reader's imagination working so we 'get' things more clearly and immediately, but also with a wider (deeper?) sense of their significance.
But there are a lot more 'goods' than that, I think. There's 'good' as in every word earning its keep on the page, nothing slack, no awkward rhythms or stumbling sentences, no turns of phrase which have lost their mojo, nothing you could cut without weakening it, nothing missing that might strengthen it.
There's 'good' as in ticking all the technical boxes that the more narrow-minded creative writing teacher and course says you should tick. Which some excellent writers can do, and still write great stuff. But the chances are that using techniques purely to show you can will knock your writerly compass off-course for whether the story actually works. As Charles Rennie Macintosh said: "There is hope in honest error, none in the icy perfections of the mere stylist", and it's written above the door of the Glasgow School of Art, which he built.
There's 'good' as in faithfulness to the dialect and/or register of the characters' voices (voice in the broad sense, not just dialogue) where it's original and where it's clichéd; vivid and authentic in how it's written; faithful to the narrative voice that the writer's chosen to use.
Or 'good' as in choosing, using and arranging words in a new way. It might be rhythm, vocabulary, imagery or syntax, but the hundreds of thousands of tiny decisions about these can add up to many different new ways: spare and tough, rich and baroque, fastidious and contemplative, off-the-wall and hilarious... but it's something original which makes the words sing in the reader's ear, and conveys the story and its meaning with greater richness and depth.
I could parallel each of these kinds of 'good' with the equivalent at novel-scale, but I won't because you're quite capable of doing that for yourself.
I think we probably all have particularly different tastes about this last kind of 'good', and arguments will rage forever along what one might roughly call the Hemingway-Joyce divide. It's the originality of such writing which raises it, at its best, to art, but therein may lie its commercial downfall. We all need some aspects of a novel to be familiar, so that we can cope with the originality of other aspects. The more that's original, though, the harder the reader has to work to understand the basics, and then get all the more out of it that there is to be got. One of my tests of a good book is whether I get more out of it each time I read it, but that implies that I didn't get everything first go. Some readers hate that feeling, and that's fair enough, while others love the sense of yet-unplumbed possibilities.
What's a pity is when readers assume that such a writer's trying to show off, or being difficult deliberately (though there are some 'literary' writers one might not acquit of that crime) rather than honestly following her/his creative impulse. And, equally, it's a pity when readers with a strong taste for originality and obscurity turn up their noses at all 'commercial' fiction, as if great story-telling and good craftsmanly writing can't exist within at least some foil-embossed covers. Both kinds of reader are missing a lot.
20th August 2019: ETA to add that since this post, I've thought more about what there is to be got from picking up a bad book - if, temporarily, you think about how it might be a good one.