Christopher Schanck compares his iPhone to his Droid X, and in doing so, highlights the interface paradigm introduced by the Android back button:
You know how every Twitter app on the iPhone has its own built in browser? And how each embedded browser works almost, but not quite like, all the other embedded browsers? With each embedded browser having different buttons, different scaling strategies, different rules for rotation? All of this exists to prevent you from having to leave your Twitter app and go to Safari, which would render the link properly. Going from TweetDeck to Safari is a big deal. After you switch to Safari, when you are done reading the link, looking at the picture, whatever, then you are stuck at a precipice — you want to go back to TweetDeck right where you left off. How do you do that? Home. Restart TweetDeck. Hope it is written such that you pick of where you left off. But traversing through the Home key and the Springboard is incredibly interruptive. Indeed, leaving TweetDeck or any other app in order to go to Safari feels like a huge step on the iPhone.
Contrast that with the Android Way: just click on the link, and whatever app you set as the default handler for that action springs open. Noise around, read it, whatever. Then just hit the back button and you are right where you left off. No barrier. It truly blurs the line between applications.
I know I’ve linked to it before, but it’s really worth repeating: The In-Between Stuff Matters.