While most will agree that the iPhone's interface is pretty, the lack of customization annoys a lot of people! Sure, you can drag app icons around until you're blue in the face, but otherwise you're stuck with the pre-programmed home screen. Android, on the other hand, is completely open-source, which means it's open to a whole lot of customization. OpenHome is a customization app for Android phones that supports custom skins, icons, and fonts, including many already designed and available in the Android Market.
Besides appearance, OpenHome supports other tweaks like widgets, auto-rotate, and keyboard enhancements. The app is free from the Andorid Market; just search for "Open Home" to get the app and view all available themes.