I'm having a hard time defining it.
I suppose you might compare it to a social venue that's open to the public, as opposed to one with a membership.
The investment of a subscription or membership, at the risk of sounding snobbish, "keeps the riff-raff out".
To put it a little better, the subscription acts as a sort of firewall, or buffer, to ensure that the only people playing the game are those that are interested in the game enough to invest money to play it.
You'll find a lot of F2P games are filled with a rabble, who are just there for a fix of fun, or to "own", or cause trouble.
You could say, almost, that many of the things people lament about UO becoming of recent years would become far more pronounced.