The mechanics have to be good - I shouldn't rolling my eyes at all the problems in a book. But the narrative has to be great. If the book is well-written, but I don't care about the characters, or the conflict, or what they risk losing in the conflict, then I'm going to get bored and stop reading the book. And popular books reflect this: Harry Potter, Twilight, and the Hunger Games are all only decent in the mechanics category - the first Harry Potters especially. But even the densest people can realize that what they like about Harry Potter is the characters, or they like the love triangle in Twilight, or the story of the Hunger Games.
When I write, I tend to not worry so much about how its written in terms of mechanics anymore. My language is sparse, and the only description I really pay attention to is the description that gets in the way. I also take solace in the fact that I may not write like Tolkien, or _____ (insert great descriptive writer here) but my book could still be loved by millions.
At the same time, there is one important aspect of writing that I would say falls under Mecahnics (though it could be narrative too) and that is the voice. Publishers and readers want voice in what they read - otherwise it's just a flat, business-like telling of events that gets you from point A to B. Voice is established through word choice, the structure of sentences, amongst other things, which is why I say it's a mechanic. In my current novel, I know that _____ would never say some of the sarcastic things another character says, _____ speaks formally that I can never include a contraction, and _____ has such an interesting voice that you could easily tell it's him speaking without the speech tag. Above all, I think drastic experiments in voice are the most noticeable aspects of a text - look at A Clockwork Orange, or anything by a foregin author (usually you can see, if they've written the English version themselves, the voice that is characteristic of their country of origin).
Above all, I think voice is probably the most difficult thing to learn as a writer. It's something I think I have to work on outside of dialogue.
Ignore everything I tell you not to do if you can do it well.
HAVE YOU CRITIQUED ANYTHING LATELY?
What is the problem?
Why is it a problem?
How can the author fix the problem?