I have only been to the Blue Mountains on a daytrip from Sydney, but I agree that this area is really beautiful (unless you get caught in one of the evidently commonplace bushfires...) and probably the best daytrip out of Sydney that one can make. On the surface, the Blue Mountains look like your average European hill chain - the term "mountains" gives them too much credit -, but their flora and fauna make them unique and fascinating, and I believe the rare flora was one of the main reasons the site was inscribed on the WH list. Nice hikes and quaint villages, too, although I think the place can get crowded on weekends.