There’s a really interesting article on what makes the Letterpress experience so successful.
The key points which I think apply to any budding application or website (where applicable) are:
Re-used metaphors: The game isn’t unique, so it doesn’t have to introduce a completely new set of rules and concepts to the user. As the article says,
This makes the rules of the game easy enough to understand and get started quickly, and hard enough to master to feel challenging.
Branding and navigation. This is obvious only once you see it for yourself. Instead of having the standard navigation bar to remind people what the name of the app is, it uses the wait spinner as an opportunity to use the logo. I think that transitions (used carefully) can be a really nice opportunity to brand the application as well as free up static space in the main activity screen
Payment nag screen. Asking people to buy the application is always a tricky problem. Asking at the end of a game is a good place – people have already seen the value proposition for themselves and will hopefully be more inclined to do so.