Just curious, who has actually played a F2P mmo on an at least semi-committed schedule?
*Raises hand* I have. For starters, the in game community does change a bit. With thousands, if not millions, of "throw away" accounts that get made naming conventions get shot. By this I mean expect to see "1337 5p34k" style names every where. With the unique name limitation, this tends to be the first casualty of converting to F2P.
First point five, the in game chat does start to suffer. If done right, it won't be too bad. But you will see more crass in-game chat.
Next, if the game doesn't start F2P (especially if it is not announced to go F2P at launch), a switch to F2P "mid-stride" is an acknowledge of failure. Name any game you want that switched to F2P, and you have to admit prior to they were failing pretty hard. (Some more so than others.)
Finally, for me at least, the "feel" of the game changes. I almost never see further content in regards to the parts of the game I liked. Probably because the parts I like aren't easily monetized, or are considered "un-sellable" by someone in charge.
In my experience, the switch to F2P never endears me to the game. It might allow me to keep playing some small part of it for free (thus keeping player numbers up)... But in the end I never spend any money on it after wards.