The problem with compulsory voting is that no box exists on ballot papers for many perspectives.
For example, I strongly believe that representative democracy (the common kind) is a farce and do not want to be represented by anybody but myself. The ballot papers make no room for this attitude - everything is framed in terms of the current system which is that either Bob or John will represent you.
It is analogous to the question "how often do you beat your wife?", which is loaded with the assumption that one beats his wife. Similarly, the voting systems we are talking about make the assumption that the person agrees with representative democracy.
Considering that I pay taxes and contribute to this society, it is, frankly, an injustice that my opinion is not represented in the least.