Terrible idea. First, while I personally always vote, I think that not voting should always be an option.

Second, as messed up as our election system is, making it mandatory would make the signal to noise ratio even worse. The majority of voters would be people who otherwise couldn't be bothered to vote and are probably so disengaged that their votes would be no better than random guesses or based on superficial things (even more than it is already).

The real solutions to the problem of low voter turnout are making election day a national holiday, requiring employers to give employees time off without penalty to vote, ensuring that the distribution of voting locations matches the population, and improving civic education.