The number of books I can read is limited by time as well as money; I've reached the stage in life where I simply will not read a book if I don't enjoy the experience. I start reading and if at any point I find myself saying, "jeeze, is this ever going to get interesting/come to the point/pick up the pace/whatever?" then I stop reading.
There is nothing under the sun that everyone likes, not even chocolate. Sometimes I'll love most of an author's work, but not every single book. Sometimes I'll feel like I should like an authors work because everyone says it's great, but I just don't.
The more I write, the pickier I get about what I read. If I find myself noticing things in the book, like how it's constructed, or if there are anachronisms or other errors in the setting, then down it goes.
Of course, unlike agents and editors, I don't send rejection letters. I don't even write reviews very often— generally only when I have something positive to say. If I were going to write such a letter, it would probably go something like this:
Thank you for publishing your book, INSERT TITLE HERE. I'm sorry to report I won't be reading it. The volume of my browsing doesn't allow a personal response as to what I didn't like, so I'll just say it simply wasn't a book I wanted to spend six hours reading. However, I'm sure another reader could feel differently, and I wish you all success in the marketplace.
A reader as well as a fellow writer