Rearrangement of messages in channel based on threads, forcing replies that weren't in thread out of order.
A UX that encourages replying out of thread because it's too darn confusing.
Unable to quote messages on desktop, but you can on mobile.
Unable to be signed into more than one Team. Have they never heard of consultants? At one point last year I had 4 separate Teams I needed to be signed into. Microsoft's solution have different Chrome profiles (Yep, Chrome, not Edge) for each one. My laptop only has 16GB of ram, so that didn't last long...
Super unclear UX around document viewing. I open a file in Teams and if I have write access I am instantly saving all changes. So many times I've shared a document for feedback and then had to recover the original version from Sharepoint because people changed a lot of things without Track changes on.
Based on Sharepoint. 'nuff said.