Multiple resolutions
Poor Tools (Eclipse is clunkier than Xcode, but there is no visual layout tool at all like Interface Builder for iPhone)
Hand Coded Layouts in XML
Multiple OS Platforms (Android 1.5, 1.6, 2.0 and 2.1 are all live in the wild and are not user upgradeable past the version they have).
Varied Screen Sizes (And no, you don't have to specifically account for this in the iPad, your iPhone app will run just fine).
In addition to those fundamental issues, currently the apis:
In-App purchase on android is a bit clunky for some programs (you are buying a separate app on the android market)
Sound is horrible, just horrible
Video is poorly documented, not very standardized, and not easy to use. People who write major frameworks have trouble getting documentation on good ways to make good videos (I know, I mailed them the FFMPEG parameters required to do streaming video).
And lastly, it looks like the G1, the flagship phone, has been mis-specced, and will likely never get the 2.x line of android OS releases (that's what droid runs now).
Because there is a variety of different hardware, screen resolutions, non-touch devices etc that all must be handled by the software, making it more complicated.
