Myself, and a cohort of 30 other MSGIS students at CSULB all did an entire class last semester with Pro. What we dislike the most is the Microsoft style ribbon. There are several reasons for this. For one, it is HUGE! It takes up too much real estate. Once you open an attribute table the map is tiny. When I used it with dual monitors I tried to drag the attribute table to the other monitor to get some real estate back, but that made the table crash. Also, toggling between tabs in the ribbon adds unnecessary clicks. Sure, that is just one small click, but when you do it thousands or millions of times a year it adds up to wasted time. The ribbon can only be opened or closed. At minimum it must be changed so it can be resized.
Finally, most of us have spent 6 to 8 years of our lives and 50,000 dollars or more to learn ArcMap, or have been using it for years at work. We do not want to start all over.