Here is what I do not like about the new Youtube UI:
1. The gray bar at the bottom of video pages. = I never use it. I prefered the simple Playlist list on the right side of the webpage, and the Related Videos underneath it. Both were to the right of the video, out of the way. It did not interfere with anything, and was there when you needed it. Now the gray bar uses up screen space, which is at a premium on small-screened netbooks and such. It loads slowly as well, and interferes with the video itself.
2. The newer "Like"/"Dislike" ratings system. = The older "star" system was much more flexible, and you were actually able to rate a video somewhat accurately (4 stars=very good, but not the best video ever, 2 stars=Not a good video, but it does have it's good spots, 1 star=garbage video). Now I must choose to "like" a video, even if there are a few weak spots, or "dislike" it, even if I enjoyed a moment or two. This is not a good ratings system. It could work as a personal preference, in conjunction with a star rating system (4 stars+dislike = Good videography, but the subject matter is not to my liking).
3. The second gray bar when the user clicks on his/her username at the top of pages = The existing 6-item menu was much simpler to access commonly-used pages (I believe it was "My Channel", "Account Settings", "My Favourites", "My Videos", "Inbox", and "Subscriptions". Simple, and easy to access what users access. The new gray bar, again, is slow to load, and difficult to access certain pages (Probably the most known is Favourite Videos list). When I favourite a video, I want to be able to access it easily, from a list. This took 2 clicks of the mouse to access Favourite Videos (Click on username at top of page, select "Favourite Videos"). The list made this easy since it was simple (6 well-sized, simple buttons) and easy to access (clicking the username at the top of a webpage brought this "menu" up). Now it takes many more clicks to get to the Favourite Videos list, the Uploaded Videos page, and Inbox.
4. The Video Player = The original Youtube video player, while somewhat simple, was easy to operate, light to load, and unobtrusive. It also defined Youtube, it's look quite iconic. I also recall, I think it was in 2008, my satisfaction upon discovering that the player controls disappeared in full-screen mode (previously, the player remained on-screen) and that clicking the video could make it pause or resume playback. I was happy with the video player: It worked, it was light, and it was small enough to not take up much space on the screen. It was then to my disappointment as I discovered the new "improved" video player. Bulky, large, and unsightly, I immediately disliked it. Even now, if I had the choice, I would switch to the original video player, which is still lurking on a few pages (Video Edit page).
5. Forced Design Changes = Perhaps because of the last 4 points, I have noticed that users have no ability to "opt-out" of new design choices. Even if a new feature is immediately applied across the board to all users, at least let these same users disable or choose not to use the new feature. Then all the users at least get to try out the new feature, and decide for themselves whether they want it or not.
Now I may not be an expert in webpage design, but I believe that a User Interface is supposed to be simple to use, attractive, and light. Youtube's UI used to be just that: Simple to use, Light on bandwidth requirements, and looked good. It has now been, for lack of a better term, massacred into it's current state: Bulky, Unsightly, and Bandwidth-hungry. A good saying for this is "Don't fix it if it is not broken", as the layout was working well, memorable, and easy to use. Then someone decided to "fix" it, and only made things worse.
Users are not content at some of the changes, as is evidenced in this forum thread, but no one seems to be noticing, at least from what we can see. There may be a discussion box, but any complaints, suggestions, etc seems to fall on deaf ears. This does not show good business practices, as a good business will take any feedback and will react. This is not a few users who dislike a certain feature, this is hundreds of THOUSANDS of users who are dissatisfied.
Another possibility that comes to mind is that the people in charge of changes to the website do not want to acknowledge their mistakes, but this is a foolish idea, as a good company will respect it's clients feedback.
Please consider this feedback, as I am only trying to help improve Youtube.