My first 35mm SLR, a Miranda Sensorex, had all the “professional” features I needed. However, due to it breaking three times within two years, I formed my current opinion as to the difference between an amateur and a professional camera.
To me, the main difference is that the professional camera is built for heavy usage. It is built to withstand multiple exposures on a daily basis. It is built to perform flawlessly even though it may be banged around. Another difference is that the professional camera is usually part of a system that allows the user to reconfigure the camera, e.g. change the lens, viewfinder, view screen, and power source in order to meet the specific requirements of the assignment. A professional camera also tends to be expensive but is well worth it to those who make their living taking photos.
I have never owned or used the Nikon FM3a. I have owned and used the Nikon F, F2, F3, and F4. However, when I needed another body, I considered buying a new FM3a because based on its specifications and user feedback, I determined that the FM3a was very reliable, accepted high-quality optics, and could withstand environmental extremes. Therefore, I classified the FM3a as a professional camera.
By the way, the only reason I did not buy one was because I discovered that the digital revolution had caused used film camera prices to drop to such a low level that I could buy another F4 for less than a new FM3a.