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While I prefer a monthly subscription business model, it can't be denied that with players wanting to dabble rather than commit, the current economy, and competition from free games, the monthly subscription model can drive some players away. However, this only applies when the game itself is either mediocre, or for some reason is appealing only in small doses to a large portion of the target demographic. Neither should be a goal for any mmo developer, but as a matter of 'better safe than sorry', one developer lead or publisher may want to adopt a microtransaction model 'just in case' their mmo is not what they hoped it would be. In such a circumstance, I think it might be a good idea to outline what I think a 'smart' version of such a model would entail, and could be adapted to almost any type of mmo.
So that everyone is on the same page, I should outline the goals of a smart model. You can decide yourself whether these are worthy goals or those that would lead to profit and happiness yourself.
1. Monetary investment by a player should convey advantages, but only up to a dollar amount approximately 10-15 dollars per months worth of purchasables. This allows players to invest up to the normal amount for a subscription based game, and competition can encourage others to invest similarly. I don't see this as a problem, as long as the amount is limited as stated.
2. Investment necessary to fully enjoy the game should scale with a player's time investment or hours of enjoyment per month. Scalability is a fundamental principle and one of the most profitable advantages a microtransaction model can have, and devs/publishers should always have scalability in mind with each MT feature. You want to be appealing to players of all levels of 'core'.
3. As little should be actually free as can possibly be managed without preventing a new player from trying the game. You read that right, as little as possible should be free. Having a set of features and areas and content for free with paid addons causes a content traffic jam as time goes on where either the free features must be expanded or new players have to buy more and more. Often getting a lot of what they don't want for free and paying more than they can justify for the extras. A la carte should be universal for all aspects of the game. How features are lumped together and the number of things to be purchased separately is a matter of fine tuning.
Example MT mmorpg model:
Fantasy adventure sandbox mmo. Five races, no classes, uses skill trees. Has three empires/factions, players can invade other faction's areas, but due to soulbinding, return trips to areas deep in enemy territory take progressivly longer. Skills are chosen at creation and can be switched around, but must be releveled if switching. Each belongs to a different tree and gains experience through different activities. Trade, Crafting, Influence, Martial Combat, Several magic schools, exploration, cultivation.
New players can play free. They get one race, and pick one skill tree to learn skills from. They start with very little. Many npcs won't deal with them, because they are grubby and poor and the player must earn a reputation, which will take them a while. There are progression missions triggered by getting skills high enough, and when one is not available, certain npcs randomly generation missions similar to CoH, EVE, and some other mmos. The player can also enlist in PvP as a lowly soldier. Higher ranking players can pick the newbie to be on their squad, and similarly kick them out if it's not a good fit.
If the player wants to make life a little easier, and expand his horizons, he can purchase the ability to use more than one skill tree. The total points he can earn and skills he can learn won't change, but it will let him diversify. He can buy extra character slots. He can buy passports allowing him to bind further away from home, eventually allowing him passage into enemy territory without being under the command of another. He can enlist as an officer rather than soldier, freely able to join other squads.
Players can purchase boosts to their XP and rewards from killing/completely quests. These boosts would be relatively cheap on a per hour basis, yet provide a very sizable (2-3x) advantage, such that it would be hard to resist for anyone enjoying the game, and cheap enough not to be considered an unfair burden. The player could also purchase in-game currency to use on the market for better gear made by player-crafters, as well as allowing them to buy a home (or rather contract for someone build him one).
The player can even buy titles to deal with more finnicky npcs, as well as get lower taxes and commission fees on market use, licenses to wear normally restricted gear, etc. And finally, my favorite idea, the player can buy the ability to access various communication channels, and the right to join or create guilds, or lead alliances (each progressively more costly). These last few would be licenses that would last either a month or a certain number of messages (spam prevention) or gameplay hours (scalability). I guess I really hate spam.
In other words, the sky is the limit, that and 10-15 dollars a month. Perhaps some of it will sound like pay to win, but at worst it's pay to win against people who refuse to pay anything. I haven't mentioned a box price. In this particular example there is none. But if there were, it would come with in-game currency equal to the cost. What's the difference? The latter requires at least some initial investment by the player.
If any of this sounds shocking or horrible, remember it comes from someone who prefers everyone just pay 15 bucks a month.