What else do they use that is quite different from the rest of the world?
ckhy:
Just to expand - A number of countries use metric levels, including China, Mongolia, Russia and many of the countries that make up the CIS (ie the former Soviet republics).
I believe it dates back to the WWII era (ie. pre-ICAO), when there was a push in Europe to go metric. After ICAO came along there was a push to standardise aviation units of measurement worldwide, but the US resisted the move to make everything metric and we ended up with the mish-mash of units and measures we have in aviation today, where some measurements are metric and others imperial. Most of the communist or former communist countries chose to keep the metric level system, probably because they had lots of Soviet built aircraft which were totally metric.
The US still lags the rest of the world when it comes to metric measurements in aviation. Visibility is usually reported in feet or miles, weight in pounds, altimeter settings in inches of mercury, and, until 1996, temperature in degrees fahrenheit.