Yeah Sydney is a bit of a tourist trap, it has basically no public transport so travellers get really stuck, it's a fantastic city for those who can experience the local haunts. Cairns is a pure tourist city but it is base camp, so to speak, for most trips to the reef etc. The Whitsunday islands are your best bet for seeing the reef, go one one of those scuba boats - all crystal beaches and booze. All the reef around Cairns/Port Douglas was damaged in a cyclone earlier this year (they won't tell u that) and it wasn't that fantastic anyway. If you do go to Cairns make sure you stay at Gilligans, it's a fantastic new backpacker place in the heart of the city, all new and snazzy and nothing else compares.
Brisbane is also really bland. Most people enjoy the Gold Coast (hell, it even kept Paris Hilton amused for 3 months!) but it's expensive. If you get like a contiki tour or something similar and whiz through the main cities you'll probably see most of what they have to offer.