voting is a right, not a duty. Both the Universal Declaration of Human Rights and the UN’s International Covenant on Civil and Political Rights refer to people’s rights to “freely chosen representatives”. You say that like "none of the above" or "I don't care" are not opinions. If people who are completely indifferent are forced to choose a candidate, they will likely just choose a name they recognize, instead of someone they agree with. If you force them to go to the polling place, and give them the option to say "None of the above" then you're forcing people to go somewhere, just to say they were there.
You could fix that somewhat by simply giving people the right to mail in or use the web to contact for a "abstain" vote anytime over the week or so before the election. Once a person puts their vote in that they are abstaining it's permanently locked in as their vote, and they can't change it, but it would mean that people wouldn't be forced to necessarily go to the polling place while still requiring a vote.
Note: I'm not really sure about how I stand on the idea; voter apathy is a pretty serious problem. This is just an idea about how you might still have compulsory voting but not tromp all over people's rights while doing so. As a comparison the census (should you be chosen) isn't voluntary; if you fail to respond to them they
will send a person to your front door to collect your data, and if you refuse even that they get the authorities involved, but we don't view that as tromping all over people's rights because it's relatively unobtrusive in our lives.
Edit: Also holy ninjas, there have been over 18 new posts since I started typing this.