Nice general question!!
To me, it has to have a rattling good plot that moves along, characters that develop...and the writing has to be pleasant and easy to read.
This let's, for instance, Dan Brown out: good, page turning action, but the characters are fairly faceless and the writing just makes me cringe.
Jo Trollope on the other hand, writes beautifully, and her characters are great...but not a lot happens. I read her a lot though.
So does a book need all three elements, or are they sort of mutually exclusive; would the full banana be too much?
(no fruit jokes here, Poppy)