There are many differences between the United States, it's people and Australia and it's people.
As mentioned elsewhere, they drive on the different side of the road. That is a major difference that takes a bit of getting use to. There are other road differences mentioned elsewhere.
One of the things we noticed when we first arrived in the US is the flags. You notice them because they are different to what you are use to nbut you also notice them because they are everywhere - very patriotic the Americans, Australia are as well but not so open about it.
Americans spell words differently. They don't like U's so colour becomes color and harbour becomes harbor. Numerous other words are spelt diffently as well.
You will be confused when they say go to the first floor of a building. An Aussie will go up a flight of stairs to the first floor - not so in the USA, the first floor is the ground floor and the second floor is the first floor and so on.
The numerical date is written backwards so the 9th March 2008 is 9/3/08 in Australia would be 3/9/08 in the USA. You wouldn't think that would be a difficult change but we stare at the figures for ages trying to figure it out. There is a few days each year that don't change - like the 1/1, 2/2, 3/3 and so on.
From our experience Americans are nealry always more polite than Australians. Service is usually good at food outlets and gas stations, etc. Service can be a bit down at hotels especially the budget ones like Days Inn.
One thing we noticed at sporting events is that when someone stands up nobody yells 'sit down ya mug'. People just look around the offending person - that's weird.
People will chat to you when you are lining up for an event or a tourist attraction - that's weird too.
You don't have to line up when you got to a Sizzlers! Now that's really weird!!!!
In the US it is almost mandatory to tip. If the service is good tip, if it is bad tip less.
To turn a light on you push the switch up, opposite to Australia. Lamps can be very difficult, anyone's guess here - turn knobs if you can find them, turn in various directions and amounts - push in, pull out. You will eventually get it but bit it will take time. There seems to be dim lighting in all the hotels, not sure what this hides.