That's a distinction with tons of teeth. It's not just microtransactions - everything about Zynga-style "games" is designed to make you play more while heavily incentivizing you to pay, while normal games provide a challenge. Zynga games don't contain nearly any challenges since that discourages players, they're button-pressing for the sake of button pressing. That's one reason why King is taking over Zynga's former spot as that upcoming new free game giant - they're providing games that are legitimately challenging, and adding the incentives for players to pay. Part of the reason for that is that the games they pick to add these psychological influences to already have a high degree of randomization that they use to their advantage, and they did mash up the formula on them by adding modified game modes, but that's going on a tangent. Back to your point about the way levels are designed, that's also part of the strategy to get people to pay - you can't see the forest for the (intentionally singular) tree.