Rating a Book...When do you decide?

I was recently reading a book and within the first half of the book I realized I had made my mind up about what star rating I would give this particular book.  Then I got to thinking about when I usually decide what star value a book is.  Is it normal to judge a book within the first half?  Or is it more prudent to wait until you have completed a book to decide the rating?  Is there something that causes you to instantly dock points from a book? 

I've read a couple of books since I started pondering the answer to these questions and I've noticed that I am stuck in the pattern of determining my thoughts/rating on a book in the first half.  Now, I rarely stop reading a book if I don't find an instant connection with it, but I may skim through the book at a faster pace. 

Granted, I also generally pick up books that have a high potential for me liking it.  This is probably why I tend to give most of the books I read a 4 or 5 star rating.  But what differentiates a four star and five star book in my mind in the first half of the book?  Well, that's another difficult question for me to answer.  Every book is different, every author has a slightly different writing style so it's not always one thing that draws me to a book.  Or deters me from one for that matter.

I still can't figure out why I generally have my rating set in stone in the first half of the book, maybe you readers have some thoughts as to that... I like to say I don't judge a book by the first chapter, but we all know that the first chapter is important to get you invested in the story and it's characters.  So maybe that's why I rate a book so early on--the connection starts from page one.