Guest-writing at Megan McArdle's blog, Timothy B. Lee riffs on the growing frustration that developers have with the closed nature of the iPhone platform versus the relative freedom offered by the open Android platform. I agree with Tim that this is a big weakness of the iPhone and that--if Android ever gets its act together--this could be the Achilles' heel of the iPhone. In thinking-out-loud about how Android could get its act together, I realized that there is one area in which the iPhone is more open than Android: the user interaction paradigm.
One of the main differences between the iPhone and Android respective user interaction paradigms is that Android has two off-screen buttons--one for moving Back within or between applications and one for accessing a Menu of actions available within each application. The iPhone puts this functionality in the screen, which whether by design or by chance ultimately leaves the decision of how to implement this functionality to the designer of each application. The net result is more user interface (UI) freedom on the iPhone. This may seem trivial at first glance, but upon further inspection I think that there's actually something significant there.
Granted, app developers still have to concern themselves with Apple's Human Interface Guidelines (and straying too far from these could--based on Apple's ambiguous approval rules--result in an app getting rejected), but in practice there is actually considerable diversity between iPhone apps. The downside to such freedom is a lack of consistency, but the upside is that market forces within the iPhone App Store can function to separate the apps with bad UIs from the apps with good UIs.
So I would argue that neither the iPhone platform nor the Android platform is as free and open as a mobile platform could be. The best solution would be a platform that has no constraints on how applications are coded (like Android) and gives UI designers maximum latitude for creating compelling interactions (like the iPhone).
Image from here.