The Layout is also a very tricky part. On iOS the app is always in full screen. On OS X, there are infinite number of display sizes and the user can dynamically change the size and ratio of the window of your app.
I say 'meant to' because autolayout is far from perfect, but there's a pretty clear shift on the iOS side from a "code to a fixed 320x480 window" to a "write a UI that can adapt to different sizes gracefully". For example, apps should in iOS 7 really now be able to handle varying text sizes (and thus layout) according to user preference.