SuperFan Rae Helmsworth posed an interesting question recently on Lee Child's message board:
What are the most important components to you of a good book? Characters? Pace? Plot? Setting? Believability? None of the above? All of the above?
An excellent question and one that I consider often when I'm assessing what I've read. Obviously there are many factors that go into making a book a good read, but I think we can focus on a few of them as being the most important.
The book has to have at least an adequate level of writing -- but you can usually assume that, if the book was published by a reputable publisher. (Occasionally you'll come across something that's abysmally written, but most books are at least competent.)
So beyond that, character is the thing for me. The book has to have a compelling protagonist, someone who makes you care about them and their life, for good or ill. Everything hangs on the characters because if you don't care about the people in the story, you won't want to read it. (It doesn't matter whether you love them or hate them -- you just have to care.)
Plot is probably the second thing -- and that depends a lot on character, too. You take a character, place them in a certain set of circumstances, and see what they do. That's how plot develops, especially when there is conflict involved (and there had better be conflict involved). Character and plot together form the basis for story, so if you can nail those two elements, I'm pretty much sold.
Setting obviously plays a part, but I think it's of lesser importance. When you read a book that uses its setting especially well, it's interesting and adds to the story. But when the setting isn't done quite as well, it generally won't ruin it. There's a lot more leeway here than with character. If plot and character are the meat of the book, setting is the seasoning. You have to really mess up the seasoning in order to make the food inedible.
The same thing goes for elements like pacing, dialogue, etc. They all contribute to the overall quality of the book, but they're secondary. If a book has dialogue that's a little off, or parts that drag, it can still be a good read as long as the story itself is compelling. On the other hand, nobody wants to read even superb dialogue coming out of the mouths of poorly-drawn, one-dimensional characters.
Great writing is nice -- it's always a joy to come across a nicely turned phrase or a particularly evocative description -- but it's not as important as you might think. I'd much rather read a well-told story than a well-written one. (And obviously I'd much rather read one that's both.) It doesn't matter how beautiful the prose is; if it's in service of a lousy story, it won't hold the reader's interest.
(On the other hand, it should be noted, the overall quality of the prose is one of the factors that can elevate a good read to a great one. It would be difficult to have a great book that has only okay writing -- although I can think of some writers who are such good storytellers that they've come close.)
Bottom line, when you're talking about genre fiction, the book has to be entertaining. These are stories we read to be entertained. Genre fiction can educate, illuminate, enlighten, and all the rest, but above all it must entertain. Otherwise, it fails.