Australia has a much lower conservative religious population. I'd say over 50% still identify as religious, but more than half of those would be non-practising regardless of the religion.
There is no social stigma in any area with respect to being Athiest.
The above mean that there is no stimga regarding church attendance or non-attendance, or pre-wedlock cohabitation or procreation.
NYE: Sydney is the first major city to celebrate NYE in the world (sorry NZ, you don't spend enough). We have a HUGE fireworks display (Harbour Bridge fireworks are super impressive) and it often make international news. You can probably find past footage on You Tube.
Otherwise holidays are much less celebrated than in the States.
Christmas and Easter are both quite family-centric but its not necessarily expected that peopel living interstate will all travel to be together. Certainly not every year.
Halloween and St Patrick's Day are pretty much non-events (unless you host a party or are going to be in an Irish pub). Valentine's is tokenistic. People might do dinner or small gifts but its not any big deal.
We mark Rememberance Day on 11 November along with everyone else, but we also have ANZAC Day on 25 April which we share with NZ and is our own persoanl military rememberance day. There are services all over the country. You can read more info about it here: http://www.awm.gov.au/commemoration/anzac/anzac_tradition.asp
ANZAV Day is a public holiday, Rememberance Day is not.
We also have a public holiday for the Queen's Birthday in June, but noone cares about ceklebrating the Queen, its just a day off work.
The fianl national holiday is Australia Day on 26 January which you can read about here: http://www.australiaday.org.au/australia-day/history.aspx
It is most often celebrated with friends around a BBQ.
Australia is also generally less patriotic than the USA. We like our country just fine, but we don't feel the need to shout about it. And we don't think we're the best at everything - because we aren't, and can't be. We just strive to be better. Tall-Poppy syndrome comes into this. And it is totally still prevalent, despite being nonsensical to a degree. But how it plays out is that basically anyone who becomes successful must remain modest. WE can think you're great, but as soon as YOU think you're great, we don't like you any more.
We also have ludicrously high expectations of our sportspersons in arenas where we have come to expect the best - ie. swimming. When our men's 4x100 relay came second, the first journalist question to the final leg swimmer was "Are you dissapointed with silver?"
That's something I'd like to see change.
People generally move interstate less. America is bigger and has more cities and people seem to move a lot further away from home. Travelling interstate for college is not the norm here. And while people certainly do move interstate, it seems far less prevalent. I think generally there are less national companies with offices in each major city to warrant people needing to move for work. Most national companies are based in Sydney or Melbourne.
Australis generally doesn't have a lot of "traditions". We're quite multicultural so there's not much that is done by people across the board.
Weddings will usually have a kitchen tea (like a bridal shower but only small gifts), hens and bucks nights (same as bachelor/bachelorette parties) but how big these are varies. Some are whole weekends, some are nights out. They're more activity based than just hitting clubs these days. Not gifts at these.
Actual weddings are usually only a day event. We don't have rehearsal dinners (or if there is something, its only for the people actually at the rehearsal and its casual. IE. My in-laws had my parents and the bridal party and partners over for drinks and nibbles afterwards.) Bridal parties are generally smaller. More than 4 on each side is unusual. 2-3 is common. There might be a casual BBQ the next day for immediate family to open gifts.
Sterotypes - generally that we're backwards, uncultured, lawless, drunken uneducated farmers. But that perception is changing. And the drinking is probably not unwarranted.
I must say, I think the binge drinking aspect of our drinking culture is shifting. While it is common to celebrate (or socialise) with alcohol, that doesn't translate to getting maggoted.