Not a be all end all explanation, but its a very comprehensive discussion on why pay to win doesn't work.
If you loved Diablo 2, Path of Exile is looking very promising. Suppose to be full f2p and the passive skill tree looks simply awesome, it is enormous!
DCUO had a pretty decent F2P System, when I played it the only things on the store were cosmetics and expansion type content like new classes and missions added on to the game after it released. Not sure how it is now, that was a long time ago.
But LoL has the best system by far, despite not technically being an MMO. Doesn't take too long to unlock champions just by playing and the only things exclusive to the store are cosmetics like Skins and Icons.