[asgard1024]

Hm, thinking about it, maybe I was wrong. Maybe you can actually express "I don't care" vote in the mandatory voting system as well, by voting for a randomly selected candidate (and these contribution will average out to same votes for all candidates). But it's rather impractical compared to simple non-mandatory voting.

[IanCal]

What about the difference between "I want X, but there's no way they're getting in, so I won't bother" and "I don't want any of them"?

[gboudrias]

That's a technical issue. There are voting systems with points, and you can vote against someone rather than just for them. And you can vote for many people. I've said it once and I'll say it again, FPTP is a tyrant's idea of democracy.