Right now, it's a pretty popular option to let subscribers of free-to-play games pay a monthly fee to get extra stuff on top of the F2P experience. Players of EverQuest, EverQuest II, and PlanetSide 2 currently get 500 "Station Cash" monthly, but starting February you'll have the option to pick up an item worth 2000 -- you can miss this item, so you have to choose every month. DC Universe Online and Vanguard: Saga of Heroes will have their own changes in tow for Legendary and Gold members respectively.
My problem with games with "sub" options on top of F2P is that they ultimately don't offer as much value as a full-on subscription based game. With all of the microtransactions on top of that "premium" sub, dedicated players just end up paying more than they would for a standard MMO and get less content for it. Still, it's a fine line, because some people only want to dabble in MMOs and don't want to commit to a monthly fee -- so they won't be playing games like Elder Scrolls Online.
Hopefully in the future there will be some sort of consistent compromise, but the developers that are working on WildStar seem to be dabbling in that.
SOE Making Changes to Subscriber Account Benefits [Ten Ton Hammer]