Some high level questions to ask before decision to build an app:
1. Will a user ever come back to your app?
The trenches of the iTunes App store are a bit like the Island of Lost Toys. "Play once and delete" is the name of the game. An app should provide a service that users couldn't perform without the phone in their hands. Just delivering copy and images isn't going to cut it; the common web browser has that covered. If your app doesn't entertain, inform and provide some type of utility on a daily basis, it will be added to the towering heap of the forgotten.
2. Will the app use any built-in iPhone features?
The most successful apps utilize a combination of features built inside and outside the iPhone in unique and surprising ways. iPhone users are intimately familiar with how these features work, so why take advantage of them? Instagram, the top-selling camera app, allows you to apply cool, retro filters and flash settings to photos taken with your iPhone. It also integrates with email, Facebook, Twitter, Flickr and Tumblr so you can easily share and order prints. To do a quick check, go to "Settings" on your iPhone and run through the list of native iPhone features and think about which services you can use.
3. How will this app use motion- or touch-based interactions?
The finger swipe is the most common interaction, but there are many more motion- and touch-based gestures to consider. The physical act of touching, shaking or tilting an iPhone to perform a task is the number one reason why it's the most popular phone on the market. Physical interaction is at the core of how humans discover and learn. It's also a lot of fun.
It’s the rare app that keeps its place on the “front page” of your iPhone home screen. Figure out how to make something useful entertaining, use the phone’s features to the fullest and optimize physical interactivity…and you’ll be three big steps closer to a permanent position next to Messages, Calendar and of course, Angry Birds.