Actually, the true killer feature for discord back in the day was something much dumber, but still heavily related to on-boarding and community transference.

You could join a discord server with a single link.

Account creation could come later.

Considering the competition at its heyday was Teamspeak or Skype, the mere fact you could just actually see the hell you were getting into without some stupid ass "Hol' Up!" instantly made it popular with basically everyone who didn't even know what it was.

My account is dated June 2015 which is apparently a month after it launched, and both me and every single one of the early adopters in that channel that is still up to this day have this same story to tell. We used it because we didn't even have to login at all in the first place when we first got it.