At a fundamental level, most readers care about character and plot. They want someone they can root for, and they want that person to do interesting things. I don't think it goes beyond that for the vast majority of readers. I don't even think it should go beyond that — read for what makes you happy. This is why inexplicably crappy books sometimes succeed wildly. Ever try to read The Bridges of Madison County?
Everyday readers — the people who buy the books — don't see bad style, poorly-drawn setting, choppy line-level language, inconsistent story arcs, contradictory themes, or any of the other craft details which writers care passionately about. They might feel the book was difficult to read, or didn't make a lot of sense, but they've never had a need to develop the critical perceptions and analytical tools required to characterize story issues. That sort of graphomania is reserved for writers. It's sort of like good finishing detail in home construction. Most people won't walk into a room and note that the crown molding is sloppy, but many of them will realize that the room doesn't look quite right even if they can't say why.
Guess what? If the crown molding is sloppy, but they really like the room anyway, they won't care.
This is why Clive Cussler sells millions of books. Every writer has authors or series they can point to and say, "well, that crap got published somehow." Cussler is my bugaboo. But while Clive Cussler's style, structure and line level prose make me go bananas and want to attack him with a dull blue pencil, millions of readers care passionately about Dirk Pitt, his series character, and the real cool, complicated scrapes Cussler gets Pitt into and out of. Therefore, Cussler succeeds as an author.
The great curse of becoming a writer is that I find it much more difficult to enjoy books with only my reader's mind anymore. The great blessing of becoming a writer is that I have a whole new way to enjoy books. And sometimes a book comes along — halduncan's Vellum, bram452's A Shadow In Summer — that makes me forget I ever was a writer, and return to the reader I always meant to be.
The rest of you guys, though, I always got something to say to you about your books and stories. If I ever trap you well enough to turn off your writer brain, great, otherwise I expect you always got something to say to me about mine.