One thing I've always wondered... how well do Imperial units scale? For instance, how do you measure stuff like the thickness of a human hair, the width of a cell or the mass of a celestial object? Metric has deviations such as mictrometres or nanometres, and prefixes to add scalability, like gigatons or teratons. Though rarely used, you can measure distances in megametres or petagrams (which might be a terribly impractical unit, hence the invention of tons).
Come to think about it, most other measurements than length, temperature and weight tend to fall within the SI system regardless of country (apart from the engine industry, which appears to prefer horsepowers over watts for some reason). You've got the candela, Newtons, decibels and all that stuff in the US too, right?