If you stay non-voter, you cannot get hired by government, cannot ask loans from government owned banks (or from the government itself), cannot get issued a new passport, cannot get issued a new id, cannot vote, cannot study in government schools and universities, your companies cannot provide services to the government, and you are considered a draft dodger (ie: for example if you want to join a private school that requires you to have completed military service, the school would be breaking its own rules if it allow you to study there)
If you do that in three elections in a row (here we have elections every two years, thus this is 6 years) your document related to voting is revoked. (meaning that you become de facto stateless, although de jure you are still brazillian).
"government" here apply to all levels (example: the part where you cannot join government schools, apply to federal, state and city schools)
Also this applies to overseas citizens (you must to the embassy and vote there).
You can "justify" your vote, this is showing up into any random voting zone (not necessarily the one where you are registered) and explaining why you won't vote, but usually the only explanation accepted is that you are away from your zone (ie: if you are into a city where you are not registered as resident).
2001, 2005, and 2010 UK elections had less than 70% turn out. That's a scary 30% to 35% of people forced to tick a box.
I'm not sure having that many people randomly ticking a box is good for democracy, especially because it's probably not a random choice but determined things like position on the ballot paper.
But voting being mandatory, is a very alarming thing. Does not sound like democracy at all. What happens, when one does not vote?