Free MMO's, generally, are more time-wasters than serious games. Free MMO's have less production value and don't have as much time and effort put into them. P2P MMO's generally have more time and effort and are for more serious gamers, compared to casual.
There are some very good F2P MMO's though, DOMO and RO (because it used to be P2P) being some of them. However, you usually have better luck finding a P2P MMO that you can stick with than a F2P MMO you can stick with.