There is way too much dead space in the new version!
We need to be able to have smaller font sizes.
Remove the second line in email headers (be able to have from and subject on the same line when reading pane is on the right i.e. exactly what outlook for windows can do and what happens when reading pane is below).
The left column side bar with the folders and mailboxes needs to be able to be shrunk in width without collapsing so we can still see all of the folders in each mailbox!
In conversation view the replies should be indented! This was one of the best features of Outlook for Windows, so easy to see if someone replied to an old email rather than the latest in the conversation
I like the flat look and redesign to suit MacOS though we really need some tweaks (above) to bring it on par with the windows version Jessica_Wilczek