I think that it is more than just visual cues. Sometimes you click, but the response is slow. If you click again, or accidentally double click, or have an ill performing mouse like one of mine that spontaneously double clicks, in all these scenarios you unintentionally end your turn! I have had to make a practice of explicitly examining the button and counting to 3 before clicking it. Usually not a good sign for "ease of use". That said, I recognize that there is very little space in which to place things, and that there are a number of other objects competing for the same real estate.
I think that it is more than just visual cues. Sometimes you click, but the response is slow. If you click again, or accidentally double click, or have an ill performing mouse like one of mine that spontaneously double clicks, in all these scenarios you unintentionally end your turn! I have had to make a practice of explicitly examining the button and counting to 3 before clicking it. Usually not a good sign for "ease of use". That said, I recognize that there is very little space in which to place things, and that there are a number of other objects competing for the same real estate.