There are many non-standard things. Huge amount of animations, all of them are "user controllable". And they must be implemented not to interfere with user input.

Then the conversion itself. The goal is to type a number and one-two letters and get a relevant result. So there are many heuristics there. For example, 0.1 meter will be converted to 3.94 inches, 1 meter — to 3.28 ft and 10000 meters — to 6.21 miles. And there is much more of that.

At last we needed the app to work well on every device that is supported by iOS7. iPhone 4 is not a fast device by today standards. So optimization was a little bit challenging also.

Maybe you have any particular question in mind?