I wrote a bunch of stuff here on my experience with microtransactions, but on reflection I think a lot of it isn't directly relevant to the question. I've posted the original infodump on my blog instead.

Basically, there's more strategy to deciding how to spend the premium currency obtained through in-game methods if that's all you get. So there needs to be *some* way of obtaining premium currency through in-game methods, and there needs to be stuff you can do at any given time so you're not just stuck waiting for everything to complete if you can't pay to finish it sooner (so, not Avengers Academy).