Back in 2007, there was an awful lot of talk about how Free-to-Play games with microtransactions were not just interesting, but also a very viable direction to take. At the time, I did not disagree with this, and I haven’t been surprised to see more and more games coming out using this model. However, while I agreed with it from a developer standpoint, I was very skeptical about playing games that relied largely on microtransactions, partly for the reasons that make them so attractive for developers. After playing a few of them, my opinion hadn’t exactly improved much either, although there are a few examples of good microtransaction systems.
When it comes down to it, many Free-to-Play games are not well designed for the consumer. You can probably see that in games like Allods, which despite apparently being a very solid MMO, has been taking a lot of flack because of their microtransactions. I think this isn’t because people aren’t unwilling to pay, but rather because people aren’t willing to pay for certain things.
Don’t charge for small things that people don’t value beyond the fact that they make the game more convenient, playable, or are considered required by the player-base.
Ultimately, a lot of Free-to-Play games charge for the wrong things. That is to say, they charge for things that the player-base considers to be ‘required’ to play the game, such as the ability to get rid of massive stat-loss from death or extra bag space in Allods (sorry to keep picking on you guys, but you’re timely!).

Bags usually aren't awesome enough to charge for.
Honestly, I don’t think a bag is worth ten cents. It is just a bag. Heck, I have a hard time spending money on a bag in real life, much less a virtual one. That isn’t something that people value. It also isn’t something that adds to the game experience. Rather, it is something that they feel they need to buy to keep playing. It is a requirement to buy, not a fun thing that enhances their experience.
While many games that have items like these in their cash shops, they do tend to provide ways to get these items using in-game means. The in-game means are often gruelingly difficult to actually do. I understand why that is the case, of course. If it is just as easy to grab the item in-game as it would be to pay for it, then certainly no one would pay for it. But this highlights the other flaw:
The purpose behind games are for them to be fun and enjoyable. Your core game, without any add-ons, needs to be that. If your game is free-to-play, I shouldn’t have to engage in microtrasactions to enjoy it.
That might seem extremely unfair to the developer – and it is! A lot of work goes into creating a very polished, good, and fun game. However, I’m only going to drop money down on a game that I actually like and find enjoyable. If I have to keep shelling out real cash for upgrades that keep my character viable, I am probably not going to have fun with the core game, so I am less likely to keep playing.
I do think that there are ways to implement microtransactions correctly. While I haven’t spent overly long playing Dungeons and Dragons Online, and while there are flaws, I think part of their cash shop is a good example of Doing It Right. While they still have items that you can buy which buff your character somehow, generally speaking, their core game experience is playable and fairly fun.
While players shouldn’t have to pay to enjoy your game, they should pay for things that extend and broaden their enjoyment of the game.
Dungeons and Dragons Online provides a decent core experience. You have a character that can be all but two of the available classes and races. The things you cant be aren’t really overpowered. You can play through a fairly decent amount of quests and adventures that are supplied to you, free of charge, when you first install the game. You can, if you want, live off of that content and nothing else – although you will probably repeat dungeons often enough for lack of content, but they are, in theory, designed to be fun enough that you can repeat them. That’s fine, and I can live with that.

Wait, why would I want to pay to be like this guy?
However, if I’m a little bored of the content, I can buy a new module. A new module features new quest lines, new things to fight, new traps to work past, and new puzzles to solve. It is, effectively, a mini-expansion at a mini-price. I already pay for expansions to subscription-based MMOs, and I have a hard time saying that extra content isn’t worth money. It is obvious to me that someone took the time to craft dungeons and write plots, and it gives me a few more hours of fun times in the game. A “+5 Longsword of Ass Kicking” on the other hand may sound cool, but it doesn’t give me anything new to experience after I’ve worn out the content of the game. Why would I waste money on that?
Players need to be able to decide when they want to spend their cash on their own terms. You shouldn’t be deciding that for them. Let the players have as much control as possible.

You know it sounds like an epic adventure.
When I reach level 20 I don’t want to feel required to buy the “Bag of Carrying More Worthless Junk”, because more things drop from mobs at that level, and I’m expected to carry around a boat-load more stuff than I did a few levels before. However, I’m totally cool with purchasing the “Adventure Through the Grue Infested Caves of Darkness” adventure pack because I like the sound of it and I’d rather like to play through it. I don’t have to, but it sounds, well, fun. That’s the point of gaming after all, right? The freedom to decide when to drop my cash is important to me. Sometimes I can afford it, and other times I can’t. I also like the ability to choose what content I want to buy, and what content I don’t want to buy. I can tailor the game to my needs – which is actually really unique to Free-to-Play models.
The things you sell should encourage you to keep providing new content to your players.
It takes no time at all to create that “+5 Longsword of Ass Kicking” but it does take a while to create a new set of dungeons and quests. When you’re selling the latter, you’ll be encouraged to invest the time and effort to actually create new content for the community. When you’re doing the former, you have much less motivation to do so. Heck, I’ve played a ton of MMOs with subscription fees – which partly exist to fund live teams to keep producing new content – only to get nothing out of them until a $30 or $40 expansion comes around.
While I’m certain this isn’t a complete list, these are certainly key things that I would like to see future Free-to-Play MMOs work to implement. Also, this isn’t to suggest that Allods is a bad game, or that DDO is some sort of gaming Messiah (it isn’t). Rather, the former is just one of many games that I don’t think handle microtransactions correctly, while the latter is an example of a game that partly does it right – and partly does the same stuff that every other game seems to.
Serving up new adventure packs isn’t necessarily the end-all-be-all, but is rather one idea that I think works pretty well. What are some other ideas? Well, I’ll worry about coming up with them if someone pays me to do so. But I like being a community manager, so you’ll have to figure that out yourselves!