Disclamer: I haven't seen the Cairns WC course or ridden any "proper" WC courses (Stromlo doesn't count, Whistler hasn't held WC's), so take the following with a grain of salt.
I swear that the people moaning about Cairns are just looking for things to complain about. As far as I could tell by all the pics, vids, helmet cams and rider interviews, that track was rad. It had a bit of everything: Steep switchies, technical, rocks, roots, high speed, big jumps, and a bit of pedalling never hurt anyone either. Also important IMO, is that it drags the WC away from the same old Europe/North America series, and it's good to see it on something that isn't an alpine ski slope once in a while, even if those locations do tend to offer the best terrain. Also, the pedalling didn't look that bad either; sure the finish was a solid slog, but the uphills appeared to be rollable, at least in the dry.
As for other venues, most of those that people have mentioned don't offer anything that Cairns doesn't. Coffs, while an awesome fun track, has virtually nothing challenging and the venue struggles to hold state rounds. Thredbo has loads of potential, but would require an entire new course. Awaba and Illinbah are both cool tracks, but they don't do anything that Cairns doesn't do better and both are a fair way from any real towns. Glenorchy is good too, but would take some work to make it WC worthy.