I've been very negative on the idea of micro-transactions in the past because I see too many parallels between them and the video arcades of the past. In fact the idea of being nickle and dime to death is still very common in our culture. I've seen it pop up in commercials for banks, cell phone providers and basically any industry where people think they can charge more by splitting fees into smaller increments. The free to play MMO's out nowadays are nowhere near as bad as big businesses in using small fee to extract overall higher prices from their customers. Still I think most gamers want to avoid them the problem by sticking with the subscription model.
1) Players shouldn't be able to spend money directly to do better in the game.
3) The game has to be as well thought out and executed as most subscription MMOs.
There is one method for using the micro-transaction model which might actually meet all of the ground rules. The game Saga uses a booster pack model to let players buy additional troops and spells. Saga is unlike most MMOs in that its based on real time strategy games instead of role playing games. I haven't heard much buzz about the game which leads me to believe it's not that well executed. But I do admire their payment model which seems to relate micro-transactions to something most gamers are more familiar with, collectible card games
Games like Magic the Gathering have been using the booster pack model for almost a decade and people seem comfortable with it as a form of micro-transaction. While players can spend a lot more money to increase their number of cards its not a guarantee for success. Random luck and strategy factors in each match along with card selection. Of course this is hard to relate to most MMOs which are based on a DIKU model of getting more and more powerful items. Still it hints at how current micro-transaction models can be modified to produce something gamers are more open to accepting.