Book Talk: How Do You Determine What to Rate a Book?

I've been giving a lot of thought to book ratings lately, particularly 5 star ratings.  As some of you may have noticed from my reviews, I don't often award 5 stars to the books I read.  For example, I read 91 books last year but only rated 4 of them 5 stars.  While I read many excellent novels last year, all of which I rated 4 or 4.5 stars, I reserve 5 star ratings only for those books that I consider to be truly special, such as those with characters or story lines that I've become more than a little emotionally invested in, or that I just can't stop thinking about.  Simply enjoying a novel isn't enough to garner 5 stars from me, a book has to leave me feeling satisfied that nothing could be done to improve upon it. 

Historical fiction is my favourite genre, as well as being the one I read most often in.  As a result, it also tends to be the genre in which my standards and expectations are highest.  While any book I rate highly, regardless of genre, must be well-written, have well-developed characters and an engaging plot, I generally won't give a work of historical fiction 4 stars or above unless it also satisfies the following two criteria:

(1) The novel must evoke a strong sense of both time and place (I love attention to historical detail); and
(2) The novel must be, to the greatest extent possible, historically accurate.  I don't mind the odd liberty taken for the sake of a story, but major liberties turn me off. 

That's not to say I don't enjoy novels that don't fulfill all of my criteria for great historical fiction, because I do, it's just that my rating of them might be lower (3 to 3.5 stars).  When it comes to the fantasy genre, another favourite of mine, I look for novels that enable me to get completely lost in another world, a world that is so well-built that I have a hard time believing it's not real.  

Recognizing that everyone has different tastes, and that what makes a book a great read for me won't necessarily be the same for other readers, I'd love to know what you consider when determining what  rating to give a book?  What elements must a book have in order for you to rate it 5 stars?