Lots of tiny details that make the app look better and enhanced overall UX, but one thing I noticed so far makes it bothersome.
On Mac, the removal of the top bar (which I think included tabs if I'm not mistaken) makes it a pain in the arse to move the window around the desktop when working with other apps, which is I think 99.9% of the cases. I don't think I have ever had ONLY the email client open, and I move windows around all the time (e.g. to see and scroll the contents of my browser without switching to it so I can smoothly type on the app on which I'm active). In most cases, every window I have open has some blank space at the top, which I can use to move that window somewhere else to see whatever is displayed there, but with Supernova I almost always have to switch to it first, because the only available space is the tiny square top left, between the green Full Screen button and the toolbar. Yes, it's only one extra click, but it wasn't necessary before the change so I thought I'd share.