There are two logical reasons against compulsory voting:
First, people do not value something if it is achieved without effort. Making voting compulsory would denigrate the nature of the voting process.
Second, those who are too lazy or indifferent to vote would, in all probability, be too lazy or indifferent to research what they are voting upon.
An ill informed electorate is worse than an apathetic one.
Its a safe assumption on my part to say there is already enough uninformed mislead people casting their votes as it is. If you dont value your vote enough to go out willingly, you probably have no idea what is going on in the world. We have had enough uninformed people voting without mandatory laws to put Bush in power twice.
It's unanimous. I can't see any arguement for it. The times I havent voted were because I didn't have anybody I felt confident voting for. My lack of vote was my vote. In each case I wrote to the 3 main parties I tend to support and explained why I wasn't voting for them.
More to the point, I never vote for municipal elections because Ilive in a different city every 2 years and have NO IDEA who those people are. I have 0 interest, for better or for worse, and don't even know who wins when it's over. I do NOT want to be responsible for voting for the party with the nice colour scheme.