I'm in Australia where it is mandatory. Urmstongran yes you do show ID at the polling booth . Charleygirl15 no it's not correct that "many voting papers would be spoiled" - well not here anyway. I actually work at the polling booth at each election and handle thousands of ballots - only a very few papers are spoiled . Most people want their vote to matter .
Tweedle24 yes people can abstain from voting for religious reasons ( have to have a letter from their church) or medical reasons. Though if you are in hospital a mobile booth comes around, or you can vote by mail.
Most Australians are pretty happy to vote and don't try to get out of it. We can vote ahead of time - for the previous two weeks there are polling booths in most areas, and anyone can go along and vote which is much appreciated by many. About 30% do it that way.