It's not so much the camera controls I have a problem with, I can get used to those. It's the UI elements themselves and how you interact with them.
For example, the Parts Action Window in KSP1 was a much more efficient method of presenting and interacting with part-specific data than the monolithic Parts Manager they have now. When I right-click on a part, I want to interact with that part only. Presenting information about all parts on the entire craft is inefficient in that it forces me to concentrate more on interacting with the Parts Manager itself to process all of the data that I don't want at that moment, instead of focusing on just the data I am requesting at that moment in time. This will be especially important when designing large interstellar ships in the future.
Another example is how there is now a gap between the parts palette and the edge of the screen. In KSP1, if I didn't like a part or wanted to get rid of several parts quickly, I could simply click on them and jerk my mouse to the left side of the screen. I didn't even have to look at where the mouse was when I left-clicked to throw the part away because I knew my mouse was up against the far left side of my screen, and anywhere along the left side would allow me to left-click again to drop the part to delete it. Now, I have to pause what I am doing and place the unwanted parts in a very narrow target area in the bottom left part of the screen to delete them. This may sound like an inconsequential issue, with a difference of fractions of a second, but its the cumulative time spent needed to hit a smaller target each time I remove a part and the attention constantly being pulled away from my craft to focus specifically on the action of deleting the part. This is quite cumbersome over several hours of gameplay.