Lord of the Rings Online went free-to-play. "Free" is a questionable term, since they charge you for a fee for features you can technically live without but are still fairly important; things such as the gold cap, the ability to gain rested XP, and certain instances and PvP options require a fee. You get an enhanced version slightly above a trial, but you are still limited in what you can do in the free-to-play version of the game.
While playing a game, I want to play the whole game, have the entire experience, and not feel as if I have been shortchanged by being on a limited version. Personally having purchased some of the Mass Effect 2 and Dragon Age DLC, I would still have to pay for the added functionality. Not being able to fully advance my character and see large amounts of content would irritate me more than the cost would. Not being allowed to make use of content would make me feel like I was missing out.
I want the choice of whether or not I do this instance, raid, or battleground. Could you imagine attempting to zone into Icecrown Citadel and seeing a message that says, "Requires a V.I.P. membership"? WoW has sold us a few items for extra money that are not required, but not having a Lil' XT or a sparkle pony doesn't affect your game functionality.
LOTRO offers things like more bag spaces and removing the gold cap and even priority login for those with V.I.P. accounts. So if World of Warcraft decided to follow the LOTRO model and go semi-free-to-play, would you just play the free portion? Would you pay for the V.I.P. portion? Would you buy the other nickel and dime upgrades they have on top of that? Or would you quit WoW altogether, feeling as if Blizzard had shortchanged players by making us pay for things like bag space, PvP and raid availability?