Screen size is small that makes navigation difficult and you cannot apply the theories of desktop to make decent navigation and you leave with very few options of UI design. On iPhone decent design means possible simplicity. Multi-touch property adds some more dimensions in UI design and makes buttons more complex than ever. Scrolling is absent so your iPhone development becomes uni-dimensional and you have to go vertically always. Due to accelerometer your layout design becomes responsive or you have to create two layouts always. Animation is rare thing not only due to absent of Flash but also due to low processing power than desktop.
Battery life is too short so you are always in crunch of power. Despite all these restraints iPhone development is in demand and developers come out with innovative applications which are highly useful in business or in real life. The future of iPhone development is bright enough.
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