Unless there is an option on the ballot for none of the above, I don't believe it should be mandatory. Otherwise, candidates will receive votes from voters who don't like any of the candidates but must choose one, perhaps flipping a coin. What it does, is give a politician a false sense that so many people support him or her.
Voting should be encouraged via tax credits, perhaps something like $300 for congress / senate elections and $600 for presidential elections. This would give financial incentive to the large portion of the population that don't vote, which would help offset the overwhelming presence of wealthy interests at the polls.
Last I looked this was a democracy even in the face of having the citizens rights stripped by misinterpretation of the Constitution, being forced to buy insurance or to be fined, with the reestablishment of debtor prison or its equivalence - so no, no one should be FORCED to vote.
People should be encouraged to vote by actually making their votes count... at present, the votes only count if they tally towards the person that corporations and big money WANT them to tally towards.
If you want to increase the will of the people to be expressed through the polls, then I recommend removal of corporate sponsorship from the election process, allowing anyone to have an equally chance in the public's eye while running for any federal office (president, congress...) and who are not cherry picked and shoved down the electorate's throats, and actually require politicians to, once elected, DO what they promised to do in order to be elected... a contract if you will.
Oh, and the supreme court... term limits and elected by popular vote. That would most likely fix that bastion of cronyism.
So long as a person can choose None of the Above it should be mandatory to vote. Societies are just a collection of animals sharing the same space. We have to collectively determine the rules by which that sharing will occur.
When someone choose to not vote, they are saying "I want to be part of this society, but you guys go figure out how it should work."
We should respond, "Do your part to help us figure that out."