Rules & regs are rather different, depending on what bit of regulation you're comparing. The devil is in the detail. For example, a PPL is not permitted to be paid to fly, unlike Oz.
The licence structure is very different, with licences ('certificates' in the US) segregated by category (fixed wing, rotary, glider etc), and by class for fixed wing (single engine land, single engine sea, multi engine land, multi engine sea). Each category/class combination requires a separate flight test ('checkride'). A multi endorsement in Oz follows you as you gain higher levels of licence. Get a multi endorsement on a PPL, do a CPL test in a single, and you will also be able to fly a multi in commercial ops. Not so in the US.
Medicals are much, much easier in the US. Same for exams.
Mostly, it's a case of switching mental gears about how things are done.