DC Universe Online saw a burst in popularity after it switched to free-to-play, and has now grown to 11 million registered users. Creative director Jens Andersen recently commented on how difficult the transition was from a standard pay model, and why he didn't want to level-gate "annoy" players into paying up after the switch.
"It was really challenging, because our content progression, everything was built with a different model in mind," Andersen told Gamasutra. The solution was the "Legendary" model, in which players can still pay a subscription fee to get all the DLC and extra perks, with a more a la carte solution for the free members.
"We wanted people to have access to the entire game--everything. All of our launch content, I should say," he said. "That was important to us; we didn't want to have to say, 'Well, you hit level 14, time to pay now.' Or do this 'poke-you-in-eye strategy' to annoy you until you sub up or give us some money or purchase something. We wanted to make it more like, 'If you like this game a lot, and you want to dedicate more time to it--not pay-to-win, but dedicate more time to it, and front-load that time and progression, you can do that through various means. Otherwise, if you want to play it for free, you'd go at the pace that the game determines for you."
That locking was based on time-gating, a model similar to casual free-to-play games. Essentially, to claim your loot rewards, you have to wait a while for its lock to clear--or pay with badges. This tied into the Legendary system as well, since Legendary players get the badges as a part of their subscription.