Stamina, stamina, stamina. Until you can run for 135 minutes with two 10 minute breaks, you don't have enough stamina to consider being an unsubbed starter. Strength, to win aerial plays and hold...
One needs at least one arm ... there are pieces for just the left or right arm ... and fingers, and lots of dexterity. It's not so much physical as the pressure required to make a string vibrate is...