Emotional rant below. Actual design feedback summary here:
1. Way too much blank space. We need a much more compact option.
2. Way too many things I don't want taking up room. We need an option to turn off the right side bar and a way to hide or collapse all the default folders so we can actually see our own folders.
3. Hiding the list of mailboxes in a drop down selector at the bottom of the screen is incredibly inconvenient. At minimum, please put that selector at the top. Or have an option we can turn on that puts it at the top. Ideally, have an option for them to just be in a list like they used to be.
4. PLEASE give us a way to organize that list of mailboxes! Even if it's just automatically alphabetized, that would be such a quality of life improvement.

Promised rant: There is a lot I greatly dislike about the new look. Everything is too big and spaced too far apart and a bunch of things I don't care about are now forcibly taking up screen space with no apparent way to get rid of them (i.e., the entire right side bar). In other words, the stuff I'm actually there to access... y'know, my email... is now less accessible. I should be able to see more messages at a time in the list without having to sacrifice all my reading space. I should not have to scroll past a ton of default folders and uselessly blank space to get to my folders, at least not without a way to shrink or remove the stuff I don't need. And most importantly, the list of my mailboxes... y'know, the things I'm here to access?... should not suddenly be hiding at the bottom of the screen inconveniently in a drop down selector. Which I still can't even arrange in an order of my choosing! Not even alphabetically! Honestly, if there aren't at least some improvements to the layout options this might be what finally drives me off this platform.