• Adoption of the metric system — Liberia and Myanmar are the only other countries that haven't adopted it. (this includes Fahrenheit⇒Celcius, Feet⇒Meters, Pounds⇒kg, and Gallons⇒Liters (or cubic meters))
• Legal drinking age (map) — the US is the only Western country that hasn't reduced its drinking age to 18. Only 10-15 other countries have a drinking age of 21 or over.
• Capital punishment — the US is the only Western country that hasn't outlawed the death penalty. While the death penalty is used by many parts of the world, it's mainly confined to half of Africa, and Asia and the Middle East.
• Universal health care — the US is the only Western country that doesn't have universal health care.
• Incarceration rates — the US incarcerates a greater percentage of its population than even the most repressive regimes, and outdoes Europe five-fold.
• Trying children as adults, when it results in life imprisonment
  In its sentencing of juveniles, as in many other areas, the legal system in the United States goes it alone. American law is, by international standards, a series of innovations and exceptions. From the central role played by juries in civil cases to the election of judges to punitive damages to the disproportionate number of people in prison, the United States is an island in the sea of international law.

Other changes I'd like to see made

• changing from a winner-take-all system to proportional representation, to move away from a two-party system, and towards coalition governments
• adoption of the World Calendar, to make the calendar vastly more regular. All you have to do is set aside one more intercalary day every year, to get 364 regular calendar days. 364 = 2 * 2 * 7 * 13 = four quarters a year, each comprising precisely 13 weeks.