Ryan Heise switched from an iPhone to a Nexus S about 6 weeks ago and has been documenting the experience. After a month with Android he wrote his initial thoughts, including what he sees as the main differences between iOS and Android:
It is the difference between Apple and Google. It is the stark contrast of a polished, holistic, curated computing experience and a mess of ideas, some implemented well, some implemented poorly, some not making any sense whatsoever. It is the difference in the feel between a finished product and a beta.