I think taxes are the biggest hurdle for microtransactions. You are legally required to report taxes in _every_ country where you have customers. This means tha

I think taxes are the biggest hurdle for microtransactions. You are legally required to report taxes in _every_ country where you have customers. This means that the customer will need to fill in address information for each website they want to use which is also a high barrier. And if you run an online business it won't take long before you have to report taxes in 200 countries. It's a job on its own to administrate that! With advertisements you receive payments from a single party, and because it's a business-to-business transaction it's tax free.