The fact that they have purposefully sabotaged the mobile web interface to block you from reading new comments to push you into installing their data slurping app is up there.
I mean, yes that's stupid, but also makes the site pretty much inaccessible in the first place and I wouldn't put that under an "user interface" complain. That's also not the redesign the op is talking about. (Unless he's using his mobile browser in desktop mode... then he afaik won't run into that restriction and it's a 100% better than the old. desktop version.)
If you see a link to a post using the default user-disrespecting site, you see the article text and maybe 2-3 “hot” comments, and then a bunch of stuff made to look like the same article but which are not. I literally cannot find how to get all of the comments for a particular topic in a threaded view while not logged in on a phone or iPad browser. The “old” site meanwhile presents exactly that.