How good the game is able to hold or pull an audience. Maple story is probably one of the more successful F2P. I didn't play too far into Maple story (maybe level 40ish) so I'm not the most understanding of it, but, it seemed to be able to grip onto those kind of people who are willing to pay for F2P even though you wouldn't be able to buy a item advantage in that game. you can buy cool costumes (cool compared to what you usually have to wear...) and I think EXP boosters (but those are time savers more than a leg up on those who don't because what cant grinding do that a EXP boost could do besides make things faster) in game. What I think made the game popular compared to other F2P's is the fact that the UI (although stupid looking) was very functional and easy and the game play generally copied what the UI did... the easy part at least (although time consuming). I think, as always, game companies shouldn't try to trick people into giving them the most money by certain methods of player-base/money per person and just simply make a good game that keeps people on for a long enough time they feel it worth it to spend money on the game (like with league of legends for me. i played it enough and found it a decent enough of a game to put 30 bucks into it. I'm still playing and don't see me stopping until valve hopefully makes a better AoS).