Completely agree that, behind the US and Canada, Australia has some of the best offers. Though Australia is a long way behind the US — if you really know how to churn cards in each market, the US is probably a 5-10x return on spend over Australia.
I've heard people say some South American countries also have very lucrative offers too (IIRC Brazil?).
The big problems for the Australian market as far as I can see are:
1. Consolidation. The loss of Citi will be a huge blow given the generous sign-up offers they used to have.
2. Trend. The trend has been downward in terms of higher annual fees (very few first year free offers), lower sign-up bonuses (remember the days of 130K+ offers — none of this wait 12 months and pay two annual fees to get it), and longer exclusion periods (18 months on NAB).