Long-form copy is really slanted towards selling high-cost products, especially intangibles to less-informed markets. You start with a vision, expect skepticism, bring the viewer round to your idea and then hit them with a pricepoint that by this time is hopefully lower than they've been led to expect. They can then buy with confidence that their money is being well spent.

With impulse buys, which is what many apps are, you really don't need the middle level of countering skepticism. You are looking to encapsulate the vision of the app in a short and compelling pitch. Most apps lack the depth to provide the material for long-form copy and you may well lose target market by giving too much and losing attention of the skimmers. Furthermore, consider the negative associations many have with such type of copy, and the correlation of those people with the tech-enabled app-market.

Regardless of your opinion of Moeed's use of ShowHN for a non-product here http://news.ycombinator.com/item?id=2275703, his landing page shows about the level of detail I would expect to see in ad-copy for an app. Just try to picture how you would extend that in order to sell the concept further - if you've run out of ideas, you've hit the level you should be at.