Android's back button is a problem. A big problem.
Others have already identified this in a broad sense, but I wanted to give a few concrete examples of why I (as a guy who wants to simply port a couple apps from the iOS platform to Android) think the back button (especially) is a bad idea.
Mobile phones, and tablets especially, require a lot of UX work in the area of interface orientation. For my extremely-basic CNL app, I've spent hours tweaking little interface elements that change when the interface is rotated from portrait to landscape.
The tendency in iOS is to use a 'back' button with the label of the previous function/screen in a given app in a navigation bar at the top of the current screen. This allows a user to freely move about inside an app, and is pretty much consistent across all apps. Additionally, this 'universal back button' is always at the top left of the screen—just like a web browser.