I think, in addition, to redorchids pretty decent list, that 'well-written' is a sort of overall term for 'mature'. No clumsy syntax or awkward lump paragraphs of description. An ability to experiment as well as adhering to the plot you've got going and the characters you've set up.
Here, I had a mild rant about a well loved author, who I think is interesting, but doesn't write well. I replaced it with this: I think too, that an important word in the definition of 'well-written' is grace. Everything you write should come with intention and every tactic you use should be deliberate. If a paragraph is hurried and confusing, it should be reflecting your characters state of mind and their actions, it shouldn't feel like it's hurried and confusing because you ran out of words.