Another benefit, and one I use to advantage is that e-ink Kindles (and other e-ink devices) are essentially designed for one purpose, to enable people to read eBooks. There are few apps installed, sometimes none. There's little temptation to use the device for other purposes - they're for reading, period.
I've provided over 20 kindles to 10 schools in rural Kenya so far, they can be charged using a small (3 watt) low-power solar panel (via a battery to store the solar charge as the kindle keyboard power circuitry doesn't cope well with the raw, variable power from the panel), and they last up to a month on a charge. Newer tablet or smartphone based devices need significantly more power, more often. They also consume lots more network traffic e.g. to update the apps, the operating system, to send and receive emails, messages, etc. Kindles in particular are parsimonious with the network bandwidth and therefore inexpensive to support (3G devices are essentially free); again Tablets and Smartphones use much more bandwidth and can easily use all the paid-for data allowance of a 3G mobile connection.
So I'm not sure whether you'd consider low power and network consumption 'features' for you - for me they are key features and advantages. BTW: There are also advantages to using Tablets and Smartphones and their apps e.g. color contents rather than greyscale, and the ability to watch videos.
In case you're interested in the project here's a link to the web site where I also maintain a blog on the work we're doing http://kusaidiamwalimu.org/