I just posted a comment under a current article over at RPS, but I would like to discuss it with you in more detail.
I feel that the term "F2P" is too nebulous and contains more different models.
From the top of my head I would say there are so far at least these three models hidden inside:
From ok to unethical:
Free to try
Start playing for free, purchase the whole game if you like it.
Just a variation of the demo. Ghost Trick or Final Fantasy Dimensions on iOS.
Pay to grind
Have access to the whole game but pay a reasonable sum to gain in-game currency (XP, Gold) faster.
Tribes Ascend – an ok model.
Pay to win
Pay to overcome arbitrary and unnecessary energy and time limits stopping you from playing.
Farmville etc. An unethical model abusing its players.
Would you add some more submodels? Or alter these three?