It's harder to tell how long an ebook is than a paper book and harder to know where you are in it from the feeling of how many pages are gripped in each hand. This is a downside of ebooks for me--harder to get an automatic sense of the geography of the book, where I am in it, where interesting things I read were located, where to flip to in order to check something.

But I think there's a silver lining to the length being so hard to see. I suspect it will lead to more authors tailoring their book length to how best to express the content rather than to the market expectations of the physical size of a "book".