Well, in theory they're quite interesting. Offering the gamer extra content, that he can purchase without leaving his/her living room, is not a bad idea. It's meant to just give the player another purchase option. If you want this awesome horse armor, it's your for $1.99!
But on the other hand, gaming throughout the ages has been about earning rewards through gameplay. If I collect 100 hidden shells, I can earn a new magical sword. If I beat this game, I can unlock a new playable character. Etc. Microtransaction takes this out of the game, instead offering these things up for a real life price. In 5 years I can only hope that games continue to offer incentives to those who play the game, and not just those who buy extra content.
Perhaps the most disturbing part of microtransactions is the idea of buying entire game environments. Having the option to buy horse armor is rather tame compared to being forced to purchase entire racing tracks just to play the game. That's quite disturbing. I can only hope that gamers everywhere reject this new form of microtransactions, which should really be called macrotransactions, and tell pubishers that this form of hackery is unacceptable.