Why do your customers return? Why to they recommend you to others? Is it because you give the most thorough eye exam in town? Is it because you know how to select and fit a PAL they'll adapt to the first time? Is it because you have largest selection of designer frames?
Or is it because of the way they feel when they walk out your door?
By a factor of three, what you do is not nearly as important as how it makes people feel.
Seth Godin points out there is a difference between doing your job and providing a customer experience. What are you focused on?