The US uses FL180 as the transition altitude based on the highest terrain. I know the highest terrain in the lower 48 states is in California and is around 14,000ft. In my opinion, it is a system that works very well because on an IFR flight plan in controlled airspace, the controller always gives their latest altimeter setting in your area when you check on below FL180.
It would certainly be nice if all countries could adhere to one set of airspace rules and definitions, making our lives easier. I imagine it could sometimes get tricky to remember different terminologies and transition altitudes.