Top of the list? Diablo III's real-money auction house.
Most, if not all, of the design problems blamed on it had nothing to do with real-money transactions but were inherent in having an auction house at all (RMAH didn't even launch until a month after release) and due to a flat, linear loot system that was purely concentrated on stacking the same few stats. As a method of secure trading between players the RMAH was about as sensible a model as we could expect for injecting real money into the equation, just as real money has now been directly (and in my view excellently) been implemented into WoW via time tokens. It's not really a cash grab when Blizzard is only taking a small cut as a guarantor of a consensual two-way transaction between players.
I say this partly as I like playing the economic layers of multiplayer games and thought the RMAH added an interesting dimension that enticed me to log in (that is, until gold inflationagain, a product of poor loot designmade it unplayable for anyone who didn't establish a big stake early). And also because I pocketed a good profit from it that's funded my way through every Blizzard title since.