I wasn't trying to claim that Australia is somehow equivalent to the US in terms of what's protected (because it isn't - the US is undoubtedly more protective of speech than Australia), but more making the point that in the porn case we don't make nanny-state laws saying that X breast size is too small. Rather, content classification is taken on a case-by-case basis using the "We'll know when we see it" threshold that allows for some flexibility and common sense without needing to make all kinds of meddling regulations.

If adopted right now with no other considerations, preferential voting offers one big win for the US over the current status quo, which is that you'd be able to quantify the true level of support that the minor parties have.

If given the option of voting third-party without running the risk of vote stealing, I'm certain that tons of Americans would take that option. Right now Alice might want to vote third party but won't do it because she knows that people like Bob won't, and Bob won't vote third party because he knows that Alice won't do it. Preferential voting breaks that cycle because it doesn't matter how other people vote and allows people to just care about their own vote.

Both the Australian and US Green parties were founded in the 1990s, and the Australian Greens have been able to take seats and hold the balance of power at times while the US Green party hasn't been able to take a single seat at the federal level or in most states. I fully attribute that to the ability of preferential voting to let minor parties grow over time without fear of vote stealing.

There are also a few other things that Australia does with voting that I think make a lot of sense, but many of these don't really need preferential voting to be of value to the US:

  - All elections (local, state, and federal) are held on Saturdays. 
    I don't really think I need to explain why this is a Good Thing.

  - Voting is compulsory. For free speech advocates this is sometimes a 
    tough pill to swallow. However, compulsory voting ensures that people 
    of all economic backgrounds and living situations still turn up and vote 
    - without making it mandatory and forcing everyone to do it many people 
    simply won't be able to find the time to vote. You are still allowed to 
    cast an informal (and thus uncounted) vote if you *really* don't want to 
    express your political views.

  - You do not have to vote at a designated polling place - you are free to
    vote at any polling place on the day of the election without any 
    justification whatsoever for being out of your electorate, and you are not
    required to register for an absentee vote ahead of time in this case. Again, 
    this is a way of ensuring that as many people vote as is humanly possible. It's 
    also somewhat required here due to it being really unfair to force people to
    vote but then potentially turn them away for being in the wrong place. 
    (Electoral rolls are of course checked during counting to ensure that people 
    haven't voted at multiple polling places)

  - Funding is allocated to the recipients of first preference votes (as long as 
    there's more than a certain threshold). House votes are worth $2.60, Senate 
    votes are worth twice as much and the value is indexed to inflation. This 
    allows smaller parties to slowly build up a base over time without actually 
    winning the election to begin with.

  - We do not use electronic voting. Our elections have comprehensive paper trails
    that can be audited to ensure accuracy.

As for things we do that don't make sense with our voting system, one downside of a diverse Senate is that we have a ton of candidates. This results in something of an excessively large ballot paper! http://resources3.news.com.au/images/2013/03/11/1226594/7771...