I feel as if many sites that used to be fine and work well have regressed in usability and features and introduced unexpected behaviors and bugs since the SPA paradigm seems to have taken over. This seems especially true for many corporate, bank, shopping brand sites, etc. An example is the Capital One 360, Chase, and AMEX sites and even Reddit.

Even JustWatch.com seems like it's fairly polished, still has it's quirks. So, I just end up using InstantWatcher.com instead. Hacker News works well as is too, except the search on the bottom can act up it seems once in a while.

Edit: How could I forget, Netflix. The UI, getting to the details of a show/movie. Browsing, discovery, searching, it all feels broken and unintuitive. The DVD site last I checked was still decent. Netflix seems like the poster child for usability and regression issues moving to a SPA.