$300 *is* pointless for an Android console (especially if as you say they are going after the console crowd) when you can add $100 to what you said and get the PS4 price curve.
I wouldn't be so quick to judge the price point. There are a lot of features that, if done right, would not only make it worth it, but would make it something that doesn't exist today... and something I really really want.
One of the features that all the current streaming boxes/sticks are missing is a dedicated control interface, separate from the TV itself.
I'm know I'm in a very small minority here - my primary TV is a projector. If I want to turn on some tunes, I don't want to have my 100" display fired up wasting bulb life, lots of power, and displaying nothing of real value. My phone, while home, is usually in the kitchen charging; My computer is locked when not using it; My laptop is put usually put away... using one of those to manage a media device is awkward and inconvenient (at least as its primary means of management).
Same thing goes for TV/movies... If I just want to see what's on, I don't want to fire up everything. If there was just a little screen next to the TV, or on the entertainment stand, or on the end table, etc.. then I could use that while other stuff is or isn't running. For example, even when the projector is on, it's usually doing something already - watching a show or movie - so I don't want to switch inputs to something else just to look around for something to stream.
Speaking of streaming... why are almost all of the current devices so horribly limited? Many don't allow you to stream from other devices on your own home network. If they do, they usually require DLNA, and only DLNA. Why can't they support CIFS/SMB/NFS/etc? For example, the PS3 has WAY more than enough horsepower to be a simple mp3 jukebox, but the interface is absolute garbage for any collection more than a handful of cd's. Amazon could EASILY fix this (as could just about anyone... but if they did it right, it'd certainly make their product a LOT more valuable in my eyes).
Now add in games... who's to say this thing won't have some real guts behind it? A high end video card and some nice controllers, and they'd be able to stage themselves for some serious titles. Speaking of controllers... the current consoles only ship with ONE, and adding a new one is another $50-$60. They could break that mold as well. They could also allow any android or iphone device to act as a controller (for most titles at least), which is something no one else is doing (as far as I'm aware).
And then there's all the other comments that have been posted - Amazon has music and video content and distribution already, and a giant network and more servers at their disposal than any other console maker, and those new VM's targeted at exposing graphics... there's plenty there to offer, and a solid foundation to plug into.
Point is, there's plenty that could make this worth the price tag. It's all speculation until some real information comes out. My guts says this leak is just testing the waters. This will probably be used more as a threat to the existing consoles (ie. open up your console to us, or we'll make our own) than something they'll actually produce... but if they actually make something, I expect it to have a mix of features that don't exist right now (outside of build it yourself).