Hi Joshua
There are many differences.
Australians overall tend to be fairly laid back. They don't take offense too easily (excepting drunks).
Our pronunciation is a little different to the US, and the spelling is British English, not American English.
We tend as a nation to 'take the piss out of each other' - that is, to give each other a hard time, to make fun of others (in good nature) etc. If someone is giving you a hard time, they are usually being friendly.
Australians tend to be cynical and sarcastic, and aren't all that fussed of people who think a lot of themselves (but no one takes it all too seriously).
(these are of course, generalities)
The lifestyle does vary from City to City, and to the towns. When I lived in North Queensland, it was much more laid back than in Brisbane (Capital City of Queensland).
The Australian accent doesn't differ too much, but Sydney is the closest to the American accent, and Adelaide the closest to the British accent (so we Queenslanders sometimes wonder if they are really Australian).
Melbourne is the most Multicultural of Australian Cities, and Sydney has the most racial tension. Perth is probably the best big city (from what I hear). Tasmania is probably the prettiest state.
All that said, my Brother in Law is American, and he occasionally found it difficult, in that many Australians have a preconception of american men as being loud and arrogant (which seems to come from TV shows, tourists, and no doubt the US govt). Having been to the US, I found them very friendly and welcoming