There are a lot of reasons a book (or film) can be good, certain authors do some things really well and other less so. The best authors (Neal Stephenson springs immediately to mind) have it all at a reasonably high level. I tend to think in terms of a handful of criteria, listed below. The overall rating I assign something will not necessarily be an average of its individual scores because the whole is not merely the sum of the parts (in some cases not even the sum of the parts) and when something is done really well, it can often make up for other inadequacies.
Characters: Some characters are cool, and others are boring, and not for only one set of reasons. Some characters are made rich by 3-dimensional intricacy and nuance, others are straight-up archetypes who work in the story precisely because they are not over nuanced.
Story: This is the larger backdrop against which the tale takes place; the world, the events leading up to the start of the tale itself, the texture if you like. How these things are told or implied is as much a part of this category as the elements themselves.
Plot: What actually happens in the book; the progression of events. Whether these events are in keeping with larger scope of the tale is important; for example twists and turns are to be expected in a mystery/crime tale, linearity and predictability are expected in a classical fantasy epic.
Style: Some authors I would read even if nothing happened in their books and the characters and events they described were pure gibberish, because they write so well; their ability to lovingly mold the English language into beautifully expressive prose is often enough.
Ideas: This goes hand-in-hand with the Story category in providing a richness and texture to the work. It is most important in speculative fiction, but can apply to other genres also. Concepts that make the world of the tale different to our own but still recognisable add something that many stories can benefit from.