another really good differentiators could be fully open source drivers, including base band firmware. That's becoming an increasingly important issue, and just apeing androids driver model where everything is closed and locked to some specific kernel versions really, really sucks.
It might not attract the masses, but it will easily snag a big following among people that care about this stuff.
Privacy. And their history and non-profit status makes them hard to compete with for users' trust in that area.