Well, we can agree on that. Of course it's always hard to make general statements--there are some great dives in the Caribbean too. But I'll make one more: don't take day boats out to the Great Barrier Reef.
I enjoy the diving in southern Thailand--I have about a hundred dives there myself, mostly day boats out of Phuket, but also one liveaboard trip on the MV Fantasea to the Similan Islands, the Surin Islands, and Richelieu Rock. Almost all of the dives on a Mike Ball itinerary to the Coral Sea would be better, in my opinion. As far as budget goes, no doubt a dollar goes further in Thailand than in Australia or the Caribbean, but how about in Indonesia?