Information density, load times, speed of navigation, UX simplicity, effort required client-side...

TBH I struggle to see how the new interface could be considered better in any respect worth considering. The only good feature is, I believe, showing images by default, which you can achieve trivially with Reddit Enhancement Suite and would have not required a complete re-engineering on a SPA model.

There might well be server-side improvements that make it worth to Reddit, but from a user perspective the new UI was a straight UX downgrade; without old.reddit.com they would probably have become another Digg.

The main feed has infinite scroll on the main feed (I am not especially a fan of that). When you click a link it is a popup over the top of the infinite scroll page. Close the popup and you are at the same position in the infinite scroll list. I didn't like the new design at first, but after using it for a bit it's not that much difference. The low quality of posts combined with heavy handed moderation is far more of a problem for Reddit than the UX.

I use RES so always had that.

> TBH I struggle to see how the new interface could be considered better in any respect worth considering.

I think the new interface integrates more seamlessly with ads, so from reddit's perspective, I can see why they'd prefer it.

Mobile issues when you don't want the reddit app:

1) no "this subreddit is only available via the app" walls

2) subreddit search functionality that doesn't require jumping through hoops to execute

3) no comment reading limitations (again a push for the app)

I'm sure there's more, but these are the things that pop to mind instantly

the main thing that I dislike about it is that they try to highlight certain - relevant - comments instead of showing the comment thread as is this happens a lot when searching, and is especially annoying on mobile where it's not clear at all on how to view the entire thread. This could follow with information density in general, I think another important piece to consider is most of the people who are using old.reddit.com probably still use RES (reddit enhancement suite).

Imagine if HN looked like reddit.com. Compare scrolling through a thread to one here.

For me, it's information density.