It seems like since nobody wants to really compete w/ WOW, the whole idea of free-to-play w/ micro-transactions for extra-content thing for MMO's is getting very popular, since it helped Turbine out majorly and kept them alive and kicking...
I think a lot of it is the result of facebook games that use microtransactions. Now every console game comes with small costume packs or item packs that cost a couple bucks to download. DLC is in some cases really small and cheap, because it's not about making a lot of money per user, it's about getting those people who think, "well, it's just a couple dollars."
I spent a few weeks playing a free to play MMO with some friends once. It was Allods Online, which uses an in game item shop. You spend real money on an in-game currency called gpotatoes, which can only be spent in the item shop. Item shop items included larger backpacks, long term buff potions, tradeskill tools, permanent character buffs (every 10 levels you can get +10% damage and HP). To make it seem like you don't have to pay, each item would have a questable version, which was usually not quite as good. But when you're looking at a 36 slot backpack for $15 or a 28 slot for free, the 28 slot looks really nice. So you spend 3 days straight grinding mobs for super rare drop quest items, and then realize you only have 25% of the quest done. You get fed up and realize it's more worth your time to spend the $15 for the bigger bag. (and trust me, they do a great job of making the 12 slot bag you start with pretty useless. Tons of quest items to carry, and most quest locations are far from town and NPCs.)
In WoW, I feel the pressure of the daily grind of needing to run instances and do dailies. It feels like everything is a long grind to keep you playing. In Allods, everything was bait for the item shop. The entire game was a long string of lures to get you to buy stuff with real life money. As soon as you buy something, there's something else the game is telling you that you need. It really turned me off of the whole free-to-play item shop setup.