This issue is to cover the latest UI changes in the stable version of October. Assuming you have enabled edge updates and are on the latest build, please provide your comments and feedback on the design changes. Once accepted your comment will be added to the main summary and deleted.
My leading concern is about the feel about the October backend. The thing I enjoy most about our CMS is that it's just so simple. It's even in the slogan, we're "Getting back to basics".
With that said, here are the some things that jump out at me...
+1 on the fonts comment from @scottbedard. Everything's pretty thick and I feel like the overall spacing is a bit too much.
Screenshots in Chrome on Ubuntu:
Before

After

My main gripes so far are to do with the sizes of things.
btn-default, it goes from grey to blue.This seems to be a bug - the hover colour is grey in build 317.#cccccc and font-weight: 400).Overall, the forms look much cleaner than before. The font and toolbar sizes are what kills it for me. As a practical example, the Products list view in Octoshop used to display 22 rows. It's now down to 16.

Apologies for the essay. Figured I'd go 'round the whole backend at once instead of posting a dozen comments
There seems to be a little bug when the colors are changed in the backend:

+1 for the comments already made, particularly agree with @dshoreman.
For emphasis...
the font is just too big, list headings are now squashed for example. I have the problem that the top menu does not scroll for me unless I touch the screen but I still prefer the text on the right rather than restricting the length of the text.
I like the new tabs though, they are easier to see. In fact everything is clearer, but I think that is more to do with the contrast changes rather than the font size.
+1 for comments above
-1 for color menu icons, I like b/w basic variants used before, because it's easy to read, it's consistent and it's easy to find new icon for my next plugin which fits actual style
Would be great to implement backend UI switcher (like backend themes) and have "Old good OctoberCMS theme" as one choice.
Thanks for your feedback.
Most helpful comment
There seems to be a little bug when the colors are changed in the backend:
