While driving is easy and the roads are good, there are some things that tend to get US drivers in trouble in congested areas:
1. as mentioned, speed limits are RIGOROUSLY enforced. 3mph over the limit (5kmh) will get you ticketed
2. drunk driving is RIGOROUSLY enforced. Alcohol limit is 0.05 and there are frequent checkpoints (all cars are stopped and drivers are tested with a breathalyzer ... no cause is required to be stopped)
3. no right turn on red
4. buses (and trams) ALWAYS have the right of way. This means if you are stopped at a red light next to a bus and the light turns green, you remain stopped until the bus pulls out in front of you (if you pull forward on green, the bus will most likely crash into your car).
5. signage is more "abrupt" than in the US and less plentiful
6. toll roads in SYD/MEL that are a pain to use with a rental car
7. gas is about USD 5 per gallon so long distance driving isn't price competitive with flying for the most part