For me and the SO there are elements that are nice in the new UI, and I can be patient enough to get used to some of the changes in layouts and locations for stuff. But when you’re browsing media (movies in our case) looking for something to watch the experience is shocking.
Go into a movie to read the description, maybe watch the trailer. Decide to look for something else and where do you end up? not sat on the movie thumbnail you used to get into the detail. You’re thrown back to the main movies section, so you have to navigate down and across again to find your place. Do this a couple of times and you’ll soon get why it’s a big issue.
How that wasn’t picked up in testing I really don’t know.