It’s a cardinal usability heuristic that status messages on your website need to be clear in communicating the current state of whatever task the user is engaged with and tell them where they can go from there. But that doesn’t mean they need to be dry and dull as if Mr. Spock was doing your copy writing.
Amazon’s iPhone app offers a playful take on the standard “you have 0 items in your cart” message seen on many ecommerce websites. Since it’s obvious the cart is empty (because there’s nothing in it), Amazon uses the occasion to suggest you give your cart a “purpose” by filling it with books or CDs. It adds an element of personality at a point in the experience in which it’s OK to be playful.
You wouldn’t use this tone if a customer’s order was canceled 10 days before Christmas because the product they purchased is out of stock and cannot be back-ordered. Nor would you use it if a consumer is in a dire situation they need to recover from, like not being able to find their previously made hotel reservation when trying to confirm it online. But in no-stress situations like the obviously empty cart more websites and mobile apps could use a little bit of personality.