I don't have the time at the moment to write a lengthy post, but primarily:
1. It's built by Google, who makes their money, and built their entire business, off of selling your information to advertisers. I don't care how "open source" it is - you're still under Google's thumb, per say. Google "Play" Store, Google services, etc are still required to run most any Android device. Amazon is the only company that came close to nullifying this, but then the Fire Phone flopped, and Amazon has become even more restrictive than Apple at some things. CyanogenMod was a nice idea, but it's now dead too, from what I've heard.
2. It looks and functions like a Windows XP clone that got ported to phones and tablets. Horrid UI, boring grid of icons, matted colors. I REALLY can't stand the UI. Apple iOS still wins in this department, although I don't like the "grid of icons" here either. The UI point has become a bit more moot on the most recent revisions of Android I've seen, but I still think it's quite poorly done.