[bguthrie]

I think this is a great analogy. The reason why it works is that we tend to relate progress in one intuitive metric (distance travelled) to a less intuitive one: time. Why shouldn't 80% of the trip be done when 80% of the distance is covered? Likewise, a merely sketched-out project can look dangerously complete.

In both cases you get caught by complexity. It's easy enough to drive in a straight line for a while, and your app's login system will likely work the same as most others'. But the devil, in both the city and in your code, is in the details.