Only one poster has mentioned the biggest barrier for many people: lack of confidence (Good Soldier Schweik). Some years ago, I participated in a workshop given by an excellent singer "for people who can't sing". Only gradually did it emerge that many of the participants could sing, in at least one case outstandingly well- but that most of them lacked the confidence to do so in public. And that if they did try to perform, their tremors overcame their singing voice, at least in their own estimation.

There are of course those with the inverse affliction.