Source: No Jitter
Those of you who are as old as I am will remember the multi-year attempt to move the United States from the English system of measurements to the metric system. The most visible push came in the form of dual speed signs placed alongside highways across the country. As I recall, the top sign showed the speed limit in MPH while a lower sign expressed the same speed in km/h. The hope was that people would subconsciously work the metric numbers into their heads and one day all the MPH signs could come down without anyone caring. Well, that didn't happen, and after about seven years, those dual signs reverted back to the original expression of MPH. Total fail.
While the idea of replacing the archaic English system of feet, miles, pounds, quarts, pints, and inches was (and still is) a good one, the voluntary approach was completely wrong. There was no mandate that those dual signs would only be up for a few months before metric became mandatory. Hoping that people would magically stop using the units of measurement they grew up with was ludicrous. Canada, on the other hand, made their switch compulsory and today's Canadians are completely comfortable telling you, "It's 23 degrees Celsius out there, eh."
This brings me to IP Version 6 (IPv6). IPv6 has been around since the 1990s, and yet most networks are still predominately built around the much older and terribly out-of-date IPv4. In fact, every time I speak at a conference or users group, I ask for a show of hands as to how many folks have made the switch to IPv6. I am lucky if I get two hands out of 100 people.