My introduction to the heady world of J came earlier this year. It was nice to get lounge access, priority queues, tags on the baggage and so on. Cathay Pacific even had a sort of traffic director in Kansai, holding up a stream of Economy passengers so I could saunter ahead of them, blowing them kisses and waving. (well, not really, actually I felt a little embarrassed)
Apart from the guff on the Qantas site, the main benefits of J are that you have a lot more room, the flight attendants have time to give you better service, the seats are nicer and you generally won't have to battle for toilet time.
For a window seat dweller like me, it also means I can get up and out without having to create major disruption.
Not that you're going to have a real lot of time to enjoy the service in the hour or so to Brisbane. Still, it's a pleasant experience, and the thought of those status points racking up adds an extra sparkle to the day.
Hey golby - hope you enjoy your trip. I think Skyring makes a pretty good point - unless you are a QP/SG and have access to all the pre-flight fun then I think you will get most enjoyment out of the pre-flight benefits - lounge access, express check in etc.
Make sure you post an update on your experience when you get back (and good luck with that BNE domestic customs clearance - its not quite as fast as regular domestic)
Flying purely domestically on an international flight you are not entitled to lounge access. It has however been reported that access is often granted, so it is worth asking politely. Just remember to remain polite if refused.