It is frustrating how few Android developers make their .apk files available directly. I don't use the Google play store, and I often have to hunt around to find .apks on "dodgy" download sites, when the author clearly wants to give the app away.
What is the market penetration through other stores compared to the pre-installed Play store? - 1:10, more? The total control of Apple is an obvious problem but in practice, dominance despite an 'open market' can be just as problematic.