What is to stop people from pumping the volume of transactions via automated micropayments to a large group of addresses? This depends on the way in which # transactions are measured.
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.