When I play a MMO, there are a lot of different factors that makes me return or stay:
  • rmt spamming to a minimum: game is supervised and not abandoned by the publisher;
  • places not empty, don't want feel playing a solo game;
  • a decent story/lore/background;
  • serious endgame not gated behind paywalls (ruled out 99,9% of f2p here...);
  • useful tools for group playing (e.g., the Duty Finder);
  • friendly environment for playing first time on a mature server (you don't find yourself isolated and hopeless because everyone capped but you);

Maybe I've missed some points but there ones are sulery key factors to me.