Infinite scroll, for example, makes it easy to lose your reading progress with a simple refresh/reload. Paged UIs, on the other hand, don’t have that problem.
Reddit is especially obnoxious because the sorting algorithms are extremely volatile, and if you do scroll too far, you'll never find that particular piece of content again.
Infinite scrolling in Twitter etc. is much less obnoxious.
I'm not even sure if it's 3PA only feature but there's option to hide all posts that you've read (or scrolled past). After finding that feature now i want it everywhere there's infinite scroll because yeah otherwise it's unusable.
That feature's way less useful when you don't read each and every post in sequential order. Also sometimes I want to go back and look at something I've already read. That makes it complicated too.
I guess the new (current since 2007) design has a lot more whitespace, although not as bad as now "mobile-first" designs of the last 5 or so years. On the topic of mobile, the current fark.com doesn't have a phone-friendly design.
Seeing the original site design, which is considered "superior" by its users to the new one, makes me feel very out-of-touch with what many others consider good user experience design.
I find the original design hard to use. The post text is very small. There is no max width to the main content, which makes it difficult to keep track of where you are when text spans multiple lines. The links are on the far-side of the page from the posts/comments they relate to, without any horizontal spacing indicators, so it's very difficult to tell what lines up with what.
The only thing I like less in the redesign is that the sidebar sections are intermixed with banner ads, which makes them hard to find and get to. But overall, I found the new design a big improvement to the old one, which judging by the responses to this and the linked post, puts me in a small minority.