Ah microtransactions, the dreaded word of many MMO players in the west. Nowadays a growing number of high quality MMO's are going free to play and will be paid for by microtransactions. The latest game which is adopting this growing trend is Turbine's Lord of the Rings Online which will be going free to play this fall. This is not a surprising move for Turbine as they already have another free to play MMO in the market as well, Dungeons and Dragons Online.
Other MMO's such as Aika online are already a free to play game with a microtransaction system which will be opened soon. This is a growing trend which will most likely change the western MMO markets into one that is similar to Asia's.
However many gamers would rather pay a subscription then use microtransactions. These gamers have a personal philosophy of believing that all features of a game should come with with the subscription and players should not be given advantages due to being able to spend real money to buy ingame bonuses.
However many of these microtransactions are purely cosmetic or give extremely small advantages. Let's take World of Warcraft, the largest MMO in the world currently, for an example. In WoW there are microtransactions for small cosmetic changes, such as race, name, gender, and faction changes, ingame non combat pets, and mounts.
The largest barrier that is slowing the process of changing the payment systems to that of microtransactions is the prevailing fear among gamers that people with money will be able to simply buy their way to getting the best gear. However the direction of current microtransaction based games is going is that they will primarily be used for cosmetic changes. The only way someone can buy their way to the top is by buying gold or other people's characters, which is often against the rules of many MMOs.
In the end only time can tell if whether the the microtransaction payment method will become big outside of Asia. However, judging by the increase of microtransaction MMOs entering the western markets it appears that this growing trend may become the next main method.