Text is great. It's probably my favorite medium. You can consume a ton of it quickly, but you have to consume it actively. It's as machine-readable as it gets.
E-Ink display's are great. They're low power so you barely ever have to charge devices that use them. You can read them outside. I actually like their anachronisticly slow refresh rates, it just makes the feeling of it as an object rather than a display even more pronounced.
Hardware that runs open-source software is great. You can write small scripts to make it do what you want to. You can modify it's interface. It can benefit from a community of users and hackers doing the same.
I want an open-source e-reader for consuming text. Ideally, it's cheap, good enough, and widely distributed. There are so many interested distribution systems for text, from social networks, to blogs, and a device that can be extended to interact with all of them in interesting ways would be hella cool.
These are the features I'd want in my ideal, open-source e-reader: