What is the Most Critical Attribute of a Fly?

Flies are a-changin' these days. We're using materials that make them more colorful, more lifelike in profile, and we can even make them move like real bugs and other edible critters better than ever before.

I got to talking about the attributes of modern flies the other night with some fishing friends at a holiday party, and none of us could agree on the single most important attribute of an effective fly. Is it the color? I don't know. A plain old gray Parachute Adams works pretty well when the blue-winged olives are hatching, and I've never seen a naturally purple insect (like the deadly Purple Prince) in any river.

It must be the profile/shape. But then again, that old Adams doesn't really look like a mayfly when you sit a natural next to the fake, does it?

It must be about the action. Big fish are predators. It takes big undulating, shimmering bugs, below and above the surface, to grab the attention of that alpha trout. Without action, there is no reaction, right? So those rubber legs and wiggly wood duck feathers, soft hackles and so forth are the ticket. Then again, you can catch mighty large fish by dead-drifting a lifeless egg in the right place at the right time.

Granted, being in the right place at the right time will make any fly appetizing. That's really the trick to effective fly fishing, isn't it? And I've said often that a good drift is more important than a great cast or a perfect fly pattern every time. But I wonder, if you were to judge what makes great flies great, and others merely good (or worse), what would be the key factor?