Respectfully disagree with above.
It takes a tremendous amount of athletism to move your body and block correctly with a ball coming in at 90mph and runners on base. No ones mentioned quickness yet, both of mind and body. A catcher has to go from a squat to getting a ball 127 feet accurately and as fast as possible while still catching the ball and taking everything happening in front of him on the field at the same time. A catchers reaction time has to be the quickest on the field. A ball to SS or 2nd base still gives the fielder time to see what's going on around him, if a runners going to third or home etc. A catcher has to catch the ball and make a decision what to do in a fraction of that time. Ask a fielder to wear a hood, then find a pop up behind him, not so easy. Pick off a runner at first on your knees?, Make a throw to second before the dust settles after a collision at home. Maybe not the same type of athletism as short but athletism non the less. The question was the most difficult position, I don't think any position needs as many tools as a catcher.