I'm glad you brought this up @noxfox, it was something I was curious about, as well. (Also the additional support for colorblind persons. :3)
Not only would it be good for helping out with feedback, but similar to comments on comic pages now, it would be interesting for readers to be able to select a passage or sentence, whatever, and say, "Wow, that was really powerful wording! I craied a bit, very emotional!" or, "This was hilarious! You really hit the humor here, it was perfect!" Like leaving comments on Word documents, is what I'm kind of picturing. Or, like nox said, forget commenting, I want to like a 'page' or 'section' of this! That was so moving, I want to <3 that so the author knows they really got to my emotions!
It's hard to do something like that for books and you want to talk about it with others--you have to remember where it was, what was said, bookmark if necessary... Having this ability would be a neat feature, and I think super helpful for writers, as well. As someone who loves to write, I would love to have a feature like this for my work!
As for actually selecting it on mobile devices... Eh, I don't know about others, but I sometimes have trouble getting my phone or tablet to cooperate with me when it comes to selecting. But I still feel like this would be an awesome tool, I'm just not completely savvy when it comes to app features.