If I compare a piece of prose I think is bad--dull, unclear with ill-formed sentences and so on--to prose I think good, I think I can see a difference. But I'm also aware that not that many people can see the difference. There are many people who think that, say, The Great Gatsby and The Da Vinci Code are pretty much the same sort of thing. I think it's similar to what happens when a real musician hears that a piano is out of tune when the other listeners can't really tell. And I don't think I have particularly refined tastes; rather, lower than even I would prefer.
Sometimes people give me thrillers to read and very often a few pages will tell me that I cannot continue to read the book. There is a certain level of bad prose that I find just as annoying as the musician finds the sour note. I can't understand the source of this failure. It's not as if there were no models of wonderful prose in popular fiction, from Chandler to Le Carré. It must be an almost organic failure of sensitivity, analogous to tone-deafness. Yet such books may sell hundreds of millions of copies.
The judgement of the crowd has therefore spoken. Does this make the sour notes true? which brings up the General Problem of Quality. That is, are qualitative judgements in art a subset of the province of taste or are there standards you can apply objectively? The instinctive answer is the former, instinctive in our relativistic age, but I still like the music analogy, where a true note can be distinguished from a wrong one by precise measurement. In literature the tuning fork is literary criticism exercised over time. Tastes change from age to age, but the classics do emerge and we use these as standards against which to measure current work.
It takes some effort, however, to imitate the masters, and many writers clearly choose not to bother, since it has been proved over and over that it doesn't matter when it comes to selling a huge amount of books. Bad writers flourish, good writers pine, an old story, but it seems to be to have become worse recently, when someone like me, with fairly low tastes, can't read a cheap thriller.