Many people cite GDP to show off which country is "the richest" or use GDP per capita to show off who is more productive. I find it more interesting to look at what people end up OWNING and what they CAN BUY for that money.
As long as you calculate everything in US$ and don't adjust for purchasing power it doesn't mean anything. Yes, US salaries are often double that of Japan, but then real estate or even food prices are easily 3-5 times as high. So here's the top 30 of MEDIAN* NET WEALTH per adult ADJUSTED FOR PURCHASING POWER.
The US being way down is not surprising considering that people are notoriously indebted all the time, and buy everything on credit, but it also shows how litte their already small savings can get them.
*Median is more realistic than average here, because in the US the high number of billionaires would actually skew the results upwards, not reflecting reality very well.