In short, compared to Australia: fewer Asians (especially in universities), and much more Afro-American (some of my black friends said that black people was just fine...but why risk trouble?) and Hispanic people.
Also, the USA has a longer immigration history (they never had a White Australia policy so recently as we did, luckily), so minorities and white people are more used to mixing with each other than in Australia. There is definitely more people of mixed ethnicity in the USA.
This is not to say that peeps in Australia don't mix, of course (cripes, I don't need a deluge of e-mail about that), and in the USA there are some people who choose to associate with their own ethnic group, but's not as pronounced. Not surprisingly, to an Australian Born Chinese who does not speak cantonese, this is a Very Good Thing (not that uh, there's anything wrong with speaking a different language in front of a friend who you know does not understand a word of it, even though you are in an English speaking country).
Americans in general are a little more extroverted and forward. Further, the main minorities in the USA (Black and Hispanic) have a more open-talking type of culture than our main minority's (Asian) culture? Maybe because a lot of people who are of foreign ethnicity are more Westernised (the immigration history thing)? Again, there are of course exceptions. I'm just theorising...
As for the question about Aussie vs US ladies - Hispanics and Filipinas TYPICALLY have a noticeably different body type and faces (think: Salma Hayek, Jennifer Lopez...) to Asians. I was surprised as to how Phillipinos can look quite different to other Asians. So based on the different ethnic breakdown in the two countries - draw your own conclusions! And that's all I've got to say about that.