In ArcMap I got very used to click+holding the middle mouse button/scroll wheel to pan around the map. This feature still works in ArcGIS Pro when you've got the Select tool selected. However, when you have the Explore tool selected, the middle mouse button no longer pans. With the Explore tool selected, one may pan by left-clicking and holding, but it feels unintuitive and disruptive to my workflow when I'm panning all over the map with the middle mouse button and suddenly that no longer works because I've selected a different tool.
This may seem minor but frankly there are a lot of minor annoyances I find myself experiencing with ArcGIS Pro after using ArcMap for a decade and if it's going to be replacing ArcMap then I feel they should be addressed.