As a born in, it was many things. Having access to old publications and reading them helped me see that things weren't right in the old days. Being friends with "spiritually weak" and "questioning" JWs also helped. I also got help while in the ministry by getting questions from people in the field that I couldn't answer from the bible but had to use "the Society says" as an authority.
Most people don't listen to someone who they are adversarial to so it is unlikely that EX-JWs would influence a JW because even talking to them could get the JW disfellowshipped. Everyone has to think and put some effort into making such a huge change as that but I can say that I was definitely not close minded and I enjoyed finding out things even if they were negative.