Here's a pretty good video that describes what happens at the WAC's. Only the supremely talented and experienced competition aerobatic pilots attend these comp's, which you can imagine is a fairly costly exercise as well. Imagine the cost of having to ship your aeroplane across the world, reassemble, etc or having to hire something very similar to your own. Australia has at most sent 1-2 of the country's best aerobatic pilots to the WAC's at any one time, sometimes with a team manager to assist the pilots. The qualifying can be brutal as well. If a certain score isn't reached in the qualifying (Q), you're out. Quite a few pilots don't get past the Q. Aerobatic pilots flying at this level are typically flying 2-3 times per week minimum and perhaps 3-4 flights per day in practice prior to the comp. G-limits can be pretty extreme from say +10 to - 7/8 G. Unlimited is extremely complex (I can hardly make out some of the Aresti figures in their sequences) with many figures in a sequence and each figure often made up of factional manoeuvres stuck together like for example a half-outside loop with a negative half snap roll off the top to inverted, etc. Now, I've never done a inverted snap roll, but I believe it feels a bit like your head wants to part company with your neck - fun huh!
This could be the most expensive, physically demanding, mentally challenging and anal sports in the world! You can imagine there is also a lot of "politics" and egos at this level of competition. The Russians usually rule supreme, with the French and US teams wining occasionally.
http://www.youtube.com/watch?v=2DGAV9v4qck