1. It is a lot more customizable; in Android you can defined what are called "scenes" (or pick from a few pre-made ones) that you can arrange with widgets that serve as your main menu, along with application icons. On the iPhone there's just a big menu with application icons; on the Android I can put a little weather widget on the front page, some widgets that notify me of events inside of applications, etcetera.
2. Android is open-source, which is kind-of an ideological thing for me, but it also means that if there's a bug, some people can just go fix it rather than worrying if fixing it is an admission that it was broken, etcetera.
3. One company does not have veto-power over all the applications the way that Apple does. At least one person has spent six months on an iPhone application and faced approval delays, and there was a tussel over a Google Voice application that would interfere with the Apple and/or AT&T business model:
http://www.wired.com/gadgetlab/2009/03/developer-inves/http://www.wired.com/epicenter/2009/09/apple-told-us-in-july-that-voice-app-would-be-rejected-google-says4. Apple puts out one phone per year, and the only real option is how much memory it has. Everyone has that one phone with those features; Android allows you to select many options from many different companies.
5. And you can select from many different carriers! Sprint seemed to have some really good rates.
6. Android applications do seem more conventional and complicated, but after using both I find that Apple has an insistence on simplicity which I think is limiting.
7. The particular phone that I got (the HTC Evo) just has a lot of good hardware; a huge screen that was bigger than the iPhone, an 8 megapixel camera (compared to 5 megapixels on the iPhone), 4G capability, and a removable battery (the downside is that it takes a lot of battery power so this wasn't as necessary on my old iPhone).