What is to stop people from pumping the volume of transactions via automated micropayments to a large group of addresses?

Question

This depends on the way in which # transactions are measured.

Answer 1

There's several anti-DOS measures that counter this:

• transaction fees get expensive with large numbers of outputs

• many rapid transactions with no fees will be rejected

• small amounts (less than 0.000045) in an output are called dust, and are rejected

It essentially becomes prohibitively expensive to try to spam.