A very possible/scary scenario goes like this:
Several major European banks are big holders of Greek debt. If Greece defaults, these banks will suffer huge amounts of write-downs. Recently, major French banks were considered for a ratings downgrade. Germany and France are the 2 biggest holders of Greek debt.
Banks will be reluctant/scared to lend to one another.
There will be a global credit crunch, a repeat of the post Lehman Brothers collapse in 2008.
Ireland and Portugal will just default too. Their governments will be thinking, "Since Greece is allowed to default, why should we suffer all the painful austerity measures?" The defaults of Ireland and Portugal will spread the debt contagion to Spain and Italy. Spain holds a relatively large amount of Portugal debt.
A global credit crisis will send the already weak US economy into another recession. US consumers will start to tighten their belts, spend less and save more, companies will start to cut jobs and spend less.
As Europe and USA go into recession simultaneously, China, who depends heavily on exporting to the Western nations will suffer a hard landing. This will trigger social unrest and even the burst of the property bubble in China.
ASEAN, being largely dependent on the US, China and Europe for economic growth will feel the brunt of this global financial meltdown.
In the mean time, I will be building up my opportunity fund and emergency fund. It is time to bunker down and wait for Greece to eventually default. USA also has a massive budget deficit of her own to deal with. Strap on your seat belts and hang on tight. Turbulent times ahead.