At the given time 25% of Facebook is taken up by various buttons and controls; there's a lot you can do and a lot of places you can navigate to from Facebook's homepage. It was obviously designed for desktop, and then ported over to mobile. After a couple refreshes the app was usable, but not exactly an enjoyable experience. So "Paper" is how Facebook would look if it were driven by designers and mobile first (if you need any more evidence that this was very design-driven, look at the marketing video. Typewriters, polaroids, naked woman in bathtub, lens flares and blur... it is, in a word, "hipster.") They also seem to have knocked off Flipboard a little bit and let you see collections of other stories. We'll see how well that takes off, but my assumption is that since it was barely mentioned it will be a minor factor in the new app, and I can't see myself using it.
The big move is from a lot of buttons and toggles on a screen to a more "swipe-friendly" UI/UX. That makes sense for mobile, but it's a big change. Something like that has to be intuitive, or it's a nightmare to use.
It's important to note that in Facebook's earnings call yesterday it revealed that mobile revenue surpassed desktop revenue for the first time ever. This new app has stories big, beautiful, and in-your-face. That means that ads will be the same way, and will likely drive a premium price. Brilliant in terms of monetization from the Facebook team.