President Obama floated the idea of mandatory voting, citing Australia as one country that does it, and arguing that it would reduce the influence of money on politics. I'm not sure how much it would reduce the influence of money. It might even increase it, because with more people voting, the candidates would see it as even more important to campaign for everyone's vote.
Besides, there is a sense in which not voting is itself a vote: namely, none of the above. So what difference does it make whether one enters a booth and refuses to endorse a candidate or stays home and refuses to endorse one, enters a booth and refuses to support a petition or stays home and refuses to support one? The result is the same.
Moreover, if people don’t care enough to bother voting on their own, why would they care enough to think through the issues and make an informed choice? Do we really want this type of voter weighing in on public policy?
Finally, mandatory voting would negate the principle of choice in the very act of forcing people to exercise it. Our President loves making things mandatory: Initially, it was mandatory health insurance; now it's mandatory voting. What's next: mandating which candidates you can choose between? He could cite China's control on Hong Kong's elections as an example of that.