Well to start with, if you format a table in excel and copy-paste it to powerpoint, the formatting now disappears. The only way seems to paste it as an image which is pretty bad.
When you format the number format of the axis of a chart, you now need two steps, adding the new numberformat to a list instead of applying it directly, it serves no purpose.
When you open a spreadsheet programatically it now opens in the background of the current sheet so you have to manually look up for it instead of opening at the front as before.
When you want to save to file it is now a two steps process, where you have to select filesystem as opposed to cloud stuff then you can save. I don't use MSFT's cloud services, this is a regression to me.
When I open a spreadsheet from an email, I now have to click the edit mode at the top to be able to do anything with it.
Microsoft making it increasingly difficult to for someone to install addin or execute macros, you now have to disable many security settings. More pain for my ass, particularly if I need to do this with my colleagues computers.
Lots of small things like that, that balance the many nano-improvements that appeared over 20 years.