The new interface looks nice, but it does take a bit to re-learn where things are. Most things I do regularly are there, but the more occasional things I have to search for. Irritating, but that's just part of software upgrades.
Attachment handling in the New Outlook is extremely frustrating. Users cannot save a file to an arbitrary location on their machine - it's OneDrive only. If the attachment is a file Office knows what to do with you do get the option to open it in <Office App>, but only after it uploads the file to OneDrive. Often the app won't launch, so you're still stuck trying to figure out where (if) it saved the attachment.
You actually don't even get to pick where in OneDrive the file goes, so they get dumped in Documents or wherever Outlook decides makes sense.
In a hurry and just need to save an emailed PowerPoint to your desktop so you can get it on the screen quickly? Too bad.
Got a lovely organization system for a type of .csv that gets emailed daily? Well, you can't save directly there anymore.
I'm not aware of an email client since Pine where users couldn't arbitrarily choose where an attachment goes.