Example of details that make a UI more compelling than another!
Page corners that really follow your finger. Bookshelves that flip to the bookstore side. The flip effect is common on iPhone, usually to reveal a settings screen; except that here, the view is THICK as real wooden shelves. Useless? I would have agreed, not long ago. But I must admit this is cool. Details like that give more presence to the application, and help you immerse in it. Simple feeling to get closer, to control better, that something's alive in there. And what's more, the pleasure of surprise.
So consider bringing a bit of the physical world to your interfaces, and keep some surprises for your users. Discovering cool yet unexpected things is always rewarding. However there are 2 things you don't want to compromise:
Workflow and concentration
You don't want to distract your user's attention to the extent that they lose focus on their task. In the iBooks example above, the page animation as well as the bookshelves effect do not hurt; this happens in less than a second and doesn't make the application harder to use.
Performance
Ensure it does not eat your whole budget in development. Ensure that it runs smooth and does not kill the device's battery.
