iOS UI Proportions (from Principia Arbiter: New Visual Proportions for the iOS User Interface, via Timoni West)
One thing I’ve noticed in a lot of custom iOS UIs (particularly ones designed by engineers) is a lack of attention to proportions. As this diagram demonstrates, the standard iOS UI has a visual rhythm—a major rhythm of 44 pixels and a minor rhythm of 11 pixels to be exact. When you work within this system for your sizing, spacing, and padding, your app will generally look and feel harmoniously iOS-like, even if the UI is heavily custom. When you don’t, it tends feels subtly off (as in the iPod playlist screen example given in the article).