Exactly what constitutes “good writing” is a subjective judgment, and it can be extremely hard to separate from one’s personal taste – not the least because one is unlikely to read books one doesn’t like, and if one doesn’t read them, one can’t tell whether they’re “good writing” or not. Furthermore, there can be an enormous amount of disagreement when you get down to actual cases.
There are so many different facets of writing that can be given primacy, that what one person says is a terrific book (with maybe a minor flaw in the characterization), somebody else may say is a terrible book (with maybe a redeeming feature or two in the plot and setting). There are a whole lot of things in any given book that can be done effectively or ignored, from the “big three” of plot, characters, and setting, to finer (or more in-depth) breakouts such as dialog, narrative, clarity, structure, style, description, accessibility, and atmosphere, to really micro-level stuff such as sentence structure, word choice, grammar, paragraph breaking, and so on.
It’s also exceptionally easy for people with a little knowledge to apply one-size-fits-all rules that really are not appropriate. I’ve seen folks complain that a stream-of-consciousness piece was “badly written” because it didn’t have paragraphing or standard punctuation…take that, James Joyce! I’ve met far more folks who don’t seem to understand that a first-person narrator has to stay in character all the time – if they don’t care what people are wearing, they’re not going to describe clothes; if they’re not interested in or aware of the emotional subtext of a conversation, they’re not going to say anything much about that, either. That’s good characterization, not “bad writing.”
Speaking strictly for myself, I’d say that “good writing” is a continuum: the more things a given piece of work does effectively, the “better” I consider the writing. Note that I said “effectively,” not “right.” There is no one “right” way to do most of these things.
And my personal taste doesn’t enter into it (or at least, I try not to let it). There are books where the character are interesting and well-drawn, the plot moves, the dialog is smooth, the background is rich and deep, they’re clear and well-structured, etc., but I just don’t much like them. For whatever reason, they don’t punch my buttons. They’re still well-written; some of them are extremely well-written. They just don’t scratch the right itch for me.
From this perspective, it gets a lot easier to figure out why many popular or bestselling books are not considered “well written” by demanding readers. It’s because most of them have one or two outstanding strengths in areas (like plot or accessibility) that lots of people agree about, while being pedestrian or deeply flawed in areas (like depth or atmosphere or diction) that many people don’t care so much for (or perhaps that don’t even get noticed by some people). One or two things done very, very well may make a book sell, if they’re the right things, but they don’t make it “well-written” overall.