One of the most profound interactions I ever had was with a player I had never met before in a pug doing Onyxia during WoTLK. I was busy topping damage and telling people to be less bad. I got a tell from someone asking why I was happy to insult people, but wouldn't step up and lead the raid since I was retired from premonition. He said people would listen, and he was right. That day I started fixing the broken pugs that I stepped into.
Thanks to his comments, which took only a minute or two of his time, we ran the "Premo Pug ICC25" runs on alleria for over a year. Eventually I transferred off some of my chars with a few friends and switched to tanking to make it easier to carry runs, but I'm still leading the top two pugs on that server. His comments didn't just begin the interest in the runs though, they taught me how to organize and lead large groups of people.
I don't even remember that players name, but I owe him or her a large debt of gratitude for encouraging me to lead.
How about anyone else? Any players you've randomly encountered who started a profound change for you?