How much harder? That would be the potential deal-breaker for me.
Harder as 1/4 to 1/2 the emphasis on basic sciences, thus leaving the average student who's focused only on his coursework completely F'ed for Step 1 (Main determinant of what residency, if any, you get). Granted, if you do can teach yourself basic sciences entirely on your own time, to a level where you can perform well on Step 1, then you'd probably encounter little resistance in terms of landing a residency in primary care, or a moderately competitive field like EM or gas (There certainly aren't many Australian grads coming over to match right now, but those that do tend to be slightly more favorably received than Carib. grads, since many Australian schools are also top-notch research institutions)
I've heard your chance of having residence in the states is slim. How difficult? I do not know.. But, Australia even addresses the degree as "Bachelor of Medicine and Bachelor of Surgery" rather than "Doctor of Medicine".
This is true, although MBBS is the main medical degree recognized pretty much the world over. Surprisingly, M.D is sort of specific to the U.S, and elsewhere signifies PhD-level work in addition to a medical degree.
I'm not quite in your position OP, but I have looked into Australian schools quite a bit. They're excellent, no doubt, but not for those who intend to come back here to practice. Even staying there as an international presents its own headaches
As I'm sure you're aware, Australia is suffering from a glut of new med schools, and a shortage of internships (residencies) that is only expected to get worse. As an international, you'll get last pick. If you're lucky. Also, after graduating from an Australian school, you'd be restricted by law from obtaining a license that would allow you to practice independently for 10 years. You could still work for a hospital or something, but that's still something to keep in mind.
Anyway, people who are interested in this route should really check out the Aus/NZ section of the international forum for more info, but from what I've seen going the Australian route almost seems like a non-option, rather than a 2nd or 3rd...