2 things - ease of access
2 things. Both UX related. 1) move the 'back button' closer than the uppermost left corner of the UI. When I open a file, honestly I look for an easy way to navigate back to the main list of files - and there isn't. Its marginal, but its friction to me that is unnecessary. Try it yourself.. when you open a file and try to navigate back - it isn't intuitive to click on the hyperlink text instead of a back button.
Second thing - the 'Open Desktop app' is arguably the button I click the most. Functionality in Teams is just not at the same calibre as the desktop app - and the new update hides this in a ellipses menu.
I'm a huge advocate for Teams in my organization. I think it has such potential. The integration of products is just hard to navigate sometimes.