You think that a book that Christie approached almost as a dare to herself would end up either a successful but cold experiment or a failure, but instead it was sharpened to a knife-point.  The sense of the characters takes a little while to get going but once they're on the island it's perfectly clear that she has a full understanding of each of them, why they're there, and why they do what they do.  There's a reason everyone loves this one; there's a reason it holds a place of great esteem even though the classic detectives are nowhere to be seen and everyone has a reason to lie.  It's a marvel.