This spoiler-free-line at the last page of the novel sums it up really well: 
 "This is how the entire course of a life can be changed--by doing nothing." 
 It includes two of the most popular McEwan-motifs--miscommunication and high-scale causality--and for which some people criticise his novels. Thing is, I'm fascinated with both of those themes, so of course I'm drawn to a book that explores them, and of course I'm going to enjoy it and give it 5 stars if it is as well written, psychologically well-built as this.