This is solely based off my own personal experience, but one thing that became very apparent to me within minutes of using Paper is that the human eye (or least my eyes) has (have) been trained to read and quickly skim content vertically and they are very good at it.
I can scroll/thumb through the traditional News Feed very quickly and easily skim a post to decide if I it's something I want to spend time reading. With paper, I found it very difficult to quickly thumb through the list of stories and asses if it's something read worthy as they move horizontally across the screen.
Hmm I'm right handed but exclusively hold my phone in my left hand. So I can take notes and write while talking. Not sure if I'm the odd one or if you are - but in my case I find the paper app easy and natural to use.
Of note, Paper's demo page (Facebook.com/paper) shows left hand usage. When I wrote the post, I looked at left hand usage and it's certainly much more natural in this hand.
I had a similar thought when scrolling in Secret right after I used Paper. I'm not sure if it's because we're used to the paradigm, or if Paper's story carousel is just too damn small / unnatural, even though most of us read left-to-right.