I agree with Chris that there is a variety of fly styles and patterns, and they are interesting. When I first started to do some research, the page that Chris referenced was one of the first I found. As I looked at the various flies, what primarily struck me was not so much the differences I saw in the various patterns, but the similarities.
All the flies are fairly generic patterns, you don't see an attempt to suggest anything too specific (i.e. a may fly wing). The flies tend to have a fair basic body profile along with some hackle wraps. What is different is the angle and placement
of the hackle.
As I started tying and fishing, I thought about the various environments where I would be using the flies. Some still water lakes and ponds, some very rapid rivers, and some streams in between. I thought about what profile I want the fly to project, and used a design and materials to try to achieve the effect.
I do tie my flies using some different colors of bodies and hackle, to be honest, I think it is for my own amusement rather than that the fish care too much. What I do think is more important characteristics are size, and hackle stiffness. I personally tie alot of my flies with lessor quality hackle. Since it is less stiff than the premium hackle, it provides more movement in slower flowing waters. I also like the slightly thicker barb diameter it provides.
As with most things fish related, we all develop our own ideas and theories. Play around with some variables, go out and fish them, and see what you think. Alot may depend on your style of fishing and the way you present your flies as well.
have fun,