Occasionally, when working on your design, something will become clear to you. Something that you never realized before. Maybe nobody else has either, but more likely it's just something you haven't properly assimilated.
For me, it was a shock to realize that I didn't have to treat every character in the game the same way. NPCs don't have to have the same stats as PCs. I'm not talking about the difference between a 1st level Commoner and a 1st level Fighter here. They don't even have to have character sheets at all, if I don't want them to.
A lot of great games have already put this into place, but I didn't really get it until a few nights ago. The light bulb went on, and I said "Hey, this game isn't about these people, the mechanics don't need to apply to them!" And I was off.
How about you? If you design, have design, want to design, or even just think about designing a game, this question is directed at you. Have you had a revelation? Was there one realization that made it possible for you to go forward?