And another appeal of Android is how people can easily be exposed to such opportunities for modification. A newbie Android user, curious as to how his more experienced peers are able to pull out some awesome screenshots of their phones in action, decides to try new stuff out for himself. Maybe some widgets here and there and the occasional system apps, but he's becoming more comfortable with it. Then as he gets more familiar with the OS, he starts to experiment on it, and discovers deeper levels of customization - ROMs, rooting, Wireless Tethering, Apps2SD - a certain domino effect that people hardly ever escape.
iOS and Windows Phone 7 are certainly more polished and refined, but they're limited on what people could do to these platforms. The streamlining these systems have does not allow for any leeway for anything else other than what's provided. If there are missing features want to be added, users can't have them unless the head honchos at Apple or Microsoft decide to do so. IMO, I don't like the reliance on the parent company just to get what a customer wants; I would prefer something less restrictive and more available for options. Google gave me that with Android.