The idea of the book is to help you find your talents, build your strengths, which will in turn, improve your performance.
Building your strengths is indeed somthing that is often overlooked, as most of the time we seek to improve our weaknesses- that's a point well taken- and a good reason to buy the book. However two more things also need to be mentioned. First, why can't we work on building both our strengths AND our weaknesses? In other words, why do we have to necessarily pick just one? I feel that many weaknesses can be improved upon.
Secondly, discovering your talents and doing what you're good at may not necessarily improve your performance. Why? Because there are lots of things we're good at, but still hate to do nonetheless. For instance, I'm really good at cleaning houses and debating, but I don't like to really do either one. People really perform well when its something that they know how to do AND when there's something meaningful/important in it for them. Anyway, just some food for thought. Readers may also be interested in The Sixty-Second Motivator.