1. Clicking on a link now takes you to the comments page, instead of the destination. To get to the destination, you need to hunt down the "direct" link. This is a shitty pattern.
2. The spacing is all weird. I just don't like the design aestheticially. But it might just be "it's different and therefore bad".
3. A lot of features aren't yet implemented on the new platform. (Crossposting, at least.)
4. Some content that I want available immediately (my multis) are loaded asynchronously, forcing me to wait before I can actually go to where I want.
5. Infinite scroll can fuck off and die in a fire. It's less of an issue on reddit, where bookmarking a given page doesn't really make sense anyway, but in principle I want my content paginated so I can have a sense of how much I've seen so far.
6. Collapsing threads is now slightly more annoying than it was, although it might be another case of "new and different therefore bad".
Some of these aren't really relevant to a link to a specific thread from outside reddit.
Also the design takes up lots of screen real-estate with cutesy useless features.
Compare the traditional UI: http://tinyimg.io/i/nBQo9uw.png
with the ugly lightboxed replacement: http://tinyimg.io/i/rOS8oVI.png
Previously, I could navigate to a subreddit, relatively quickly glance at whether anything was new, without being distracted by autoloading images and videos. Now reddit looks more like ebaumsworld 10 years ago.
Design wise I’d prefer it focus on community > consumption. Card interfaces with expanded media are very consumption oriented.