Non-programmers tend to think that adding a software feature is akin to adding on to a house. Sure, it take some effort and money, but the resulting house is surely better, right? (This isn't always true, of course, but still)

In reality, adding a software feature is often like adding onto a house in the center of it, expanding the rest of the house around it to make room. Suddenly the job looks a lot tougher.

Or perhaps if you told them, it's like adding scenes to a film, they'd understand. There's a reason for deleted scenes.

How about "it's like if they took [your favorite sport] and added a new rule to it. Imagine how much all the theory, leagues, player valuations, etc. would all change."

Or slightly more precise, but less accessible—"what if your favorite fighting game got a new character? What if your favorite MMO got a new class?"