I found this one quite good.
tberman: I agree that voters should have the right not to vote for anyone, but don't feel that simply not turning up to vote is a good way to do so. In a non-compulsory election, the non-voter count is going to be comprised of those who are abstaining from voting, and those who are simply two lazy to turn up. With compulsory voting, those who don't wish to vote for any particular candidate can simply leave their ballot blank, which is known as an informal vote.
Given the difference in turnout between U.S. and Australian elections, I'd guess that a fair number of the people who don't vote in the U.S. would vote if they had to turn up to a polling place on the day.
I also think it is important for as many people as possible to vote. The people who get elected are supposed to represent the electorate. When there is a clear majority it doesn't matter much, but in a marginal seat, those missing votes could easily swing the result. In this case, people can claim that the winner does not have the support of the majority of the electorate.
As far as the U.S. gravitating towards a two party system, I'd suggest that this is caused more by the vote counting procedure than the culture. When you have a system where where voting for someone who won't get a high first preference count is equivalent to not voting, people are going to gravitate towards the parties where their votes actually make a difference (or not vote at all).
With a preferential system, you can vote for a minor party as your first preference, then number off the major parties with your other preferences. This also fixes the problem where two similar candidates might split the vote causing both to lose -- one will get knocked out, and their votes will be transfered to the next preference (which would likely be the other similar candidate). This generally leads to the least unpopular candidate winning, rather than the most popular one.