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 anachronistic 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:
Generally speaking the two most important features here are the ability to push and pull content. For example, the Kobo currently does this really well with Pocket articles. Not only are the articles pulled automatically from pocket, but I can favorite, archive, and delete them, and that's also kept in sync.
The kindle ecosystem has a feature for emailing books and pdfs to your device, which somewhat deals with getting information on there, but doesn't do anything for getting information off.