* A very opaque answer-ranking system which often promotes what I consider really crappy (e.g. factually incorrect or off-topic) answers over better ones. This is not just stupid people voting, BTW; simple ranking by vote count would be far better than what actually happens. This is something Quora themselves, or their algorithms, do.
* A cadre of popular writers who openly flaunt the rules (e.g. don't answer the question, fill posts with meme-y images). A significant subset of these "Slop Writers" are clearly there to peddle their own books with an annoying coda on every answer.
Those kind of things seriously degrade the reading experience there. I honestly don't get why Quora would allow, facilitate, and even seem to promote behavior which drives away both readers and writers. It would only make sense if people were actually paying them to have their answers ranked higher or to be allowed to break the rules, but that would be extremely hard for them to hide so I don't think that's really the case. Considering some of the other problems there, like the site generally being slow/flaky or the random addition/removal of features, I think they're just not thinking very hard about these issues.
I still go back there to read sometimes. So far, I keep finding that the same problems are still there and getting worse, so I don't expect I'll write there again.