Hard one, since Windows Phone is a paradigm shift and more of a hands-on product. You won't do it justice with a sentence (unless your friend needs a specific thing that WP excels at), or an image / short video.
If I am with a friend and in that situation, I'll just lead the sales troll over to the Atrix and ask him why the Gallery app can't pinch-to-zoom to save a life.
its the os that cut the distance between you and your friends not matter where they are you will stay in touch with them its the os that you want for your son daughter your wife its the os that you would recommend for your manager or business man to accomplish their daily tasks when they are far from their office or people who love music or for the gamers
in short sentence
its the only OS which been made for everyone