With ads every visitor/reader contributes regardless of their agreement with the content, while micropayments are more like "likes", and you'd mostly pay those who you agree with. This could lead to karma-whoring but this time for real money.

I mean, authors would have to kiss the asses of the readers and try to be as friendly to their preconceptions as possible and make them feel like they are buddies.

Most donations are done based on emotions. So now we'd have distorted content, catering to the online hivemind even more than today.