Appbot have a great post on Writing a Great App Store Description.
One of the points is about telling a story, something that many apps fail to do.
Tell a story, don’t rattle off features
Selling a story of the person your potential customers want to be is what you are trying to achieve. Are they going to be awesomely productive? Are they going to save money? Are they going to get fit in just 7 minutes? Tell me about the super human I will become using your app.