It's not as funny as you may think!
The state of California ON ITS OWN has a higher population (almost double) and a higher GDP than Australia does.
A lot of people have mentioned that the two cultures are different. As far as Americans are concerned, can it not be argued that the USA has a number of different cultures within it anyway? Absorbing a foreign nation with a different culture again would not be such a shock to the Americans I would think.
It would be quite the shock to Australians though. But there is not necessarily any reason why Australian culture would suddenly change simply due to becoming a state of the USA. Yes, laws and politics would be quite different and could take some adjusting. But I don't see culture changing that quickly.
You also have to remember that if the USA absorbed Australia today, none of the arguments that anybody is making would mean anything in say 200 years. In the future, the people then would simply know the USA to exist both on the North American continent and the Australian continent. Keep in mind that in this hypothetical scenario, Australia would have voted and asked to join the USA. We are not talking about a hostile takeover here. This would result in a distinct difference of the attitudes of people living on the Australian continent in the future.