Sorry to butt in, thought I'd add my two cents worth.
The east coast between Sydney and Cairns (the popular places in between include Byron Bay, Surfers Paradise, Brisbane, Sunshine Coast, Airlie Beach, Fraser Island, the Whitsundays, Great Barrier Reef - there are literally dozens upon dozens of places of note. The whole coast is a point of interest) is definitely the most well trodden backpacker route.
The west coast route between Perth and Broome, is similar to the east coast in terms of what you can see (sun, beaches, rolling surf, small, laid-back towns, even a reef in Ningaloo Reef) but with far fewer people. Lots of people I've spoken to say the west coast route is nicer because of its more unspoilt nature. Broome to Perth will be the last part of my year in Oz before I fly back to the UK in September - can't wait.
From personal experience, Premier Motor Service is far better than Greyhound. If you can live with their less frequent service (one service a day Brisbane <> Cairns, rather than Greyhound's three/four) you're rewarded with cheaper fares and a more pleasant journey (nicer drivers, better buses and you almost never get someone sat next to you).