what would have (physically) prevented someone from joining the armed forces in the 1940's?

the wrong sex organ!

Not to much. Due to the need for all the troops they could get, If you could talk, walk, hear, see and could be trained they could find a job for you in the military. In fact so many men had to go that the women had to go to work in the factories to build war supplies, so this proves that not to many ailments kept a male at least 17yrs old from going to the military. In a time of great need everyone had to pull together for the good of the country and the result was that all of us are able to live a good life. Wars are terrible but sometimes they are necessary. The Answer Man---Al Cote

Besides the obvious answer of quadriplegic, things like extremely poor eyesight or loss of a limb, having equilibrium imbalance that would make it hard to stand up straight. During WWII they weren't exactly picky on what you had with drafts and basically fighting on three fronts, yes I said three, most people forget about africa and italy campaign. Having flat feet or asthma wasn't going to keep you out, because there are a lot more jobs than just infantry. I hope this helped you

Quadriplegic

Flat feet. That means when you have fallen arches in your feet. That was a very typical defect they wouldn't allow , because of the amount of marching involved in the forces.

The color of his skin.