I think microtransactions are bad when the players cannot truly enjoy the game unless they buy them. For example, energy bars system implemented purely to earn money, or power boosts that give players definite superiority against others in pvp. 

Other than that, I think microtransactions are actually a good thing to the players and the game itself. First, game developers will have more incentives in maintaining and improving a game with microtransactions, because it still generates money. All players will benefit from the continuous bug-fixing and game content additions. Also, microtransactions keep the game cheaper than if everything is bought in a package. Some players might not want the additional aesthetics or some of the story content, so it will be best sold separately. 

In general, I see microtransactions as a [color=yellow]price discrimination tool[/color] used to target different consumers with different needs. It can become ugly if the game developers did not do their market research to understand their target audiences well.