It’s a really good question- and almost impossible to define an answer, although someone downthread had a really good go at it.
For me, it’s linked to:
a variety of sentence structures
accurate punctuation- I always notice when writers need to learn semi colon usage, although might just be me
I enjoy books more if I care about at least one character, but I don’t think that’s a pre-requisite for a book to be well written
I was going to say evocative imagery, but then I remembered Steinbeck, who uses very little whist being an amazing writer
I’ve therefore totally failed to define it and would be really interested to hear what others think.