For example while I like to listen to BBC and NPR news I never appreciated audio books.One of the killer features of the Echo is it's "always on" feature. This is also a little spooky because someone could be listening and it's a short hop from Apple cracking iPhones to playing back Echo's audio (There is a Google dashboard flag for disabling backing up OK Google commands so I imagine there is one for Echo.)
The second killer feature for the Echo is that it now comes in 3 varieties. (i) a puck called Dot intended to be connected to a speaker or stereo, (ii) a battery powered speaker called Tap (iii) and the original Echo. Since they are dedicated for their purpose the UX is simple and to the point.
And while Google's Chromecast and Chromeaudio are cool on their own they require specialized Chrome clients and servers to make everything function. And while my Nexus 6 is a little more portable when compared to the Dot I'm not usually telling google what I want to listen to when I'm on a plane. So the use case for Alexa and OK Google are pretty much the same.
And while Alexa is an amazing feat unto itself Amazon is about selling stuff. Their Amazon music is just not as good as Google Play Music. Google needs some better hardware and Amazon needs to play better with Google... at least I hope they learn something; it's bad enough that they share ad and search info.