You're missing the fact that it's very possible to use coercion to discourage certain groups from voting at all. Just target the groups that are likely to vote for your opponent, and you've essentially used coercion to manipulate the election.
This is a problem which would be solved, somewhat ironically, by the US adopting mandatory voting. Shift a really nominal fine onto "not voting" (like $30), and it will become politically impossible to get away with marginalizing groups from voting.
Because people might ultimately think "well my vote probably wouldn't have mattered" when the line is 8 hours long on a Tuesday, but they will definitely care when they get fined $30 - but they're not going to simply argue the fine should go away, because it will be a little reminder of "oh yeah, and you didn't get a say in your government either!"
If I were Australian, I would have a problem with it, because I am what you might call a principled abstainer. But apart from personal issues regarding perceived rights violations, there are several obvious advantages and disadvantages, similar to any aspect of election design. I'm not saying compulsory voting is game-breaking, but merely that it's a compromise that one could argue is not desirable.
You're under no obligations to actually vote in Australia, just to show up. You can freely get your name ticked off, put a line through all the candidates and write "try harder" and walk out.
Fair enough. It's hardly the worst idea I've ever hear of, nor the most upsetting to be personally (the fact that "modern" Western governments still reserve the authority to perform military conscription is far worse). I would still prefer not showing up for paying the fine.
When I lived in Ecuador long ago, voting was mandatory. The people with your views ran a spirited campaign with posters that said "VOTA NULO", vote null. I'm not sure exactly how it worked there, but in some places there's an explicit "vote for nobody" choice, and those choices are counted and reported: http://es.wikipedia.org/wiki/Voto_nulo
You're missing the fact that it's very possible to use coercion to discourage certain groups from voting at all. Just target the groups that are likely to vote for your opponent, and you've essentially used coercion to manipulate the election.