In the world of fly tying, little dialog has taken place regarding the development of a philosophy for the design and tying of fly patterns. We want fly patterns that work...period. Therefore, discussion leans heavily towards the pragmatic how-to aspects of assembling a fly. While certainly necessary, moving beyond the mere how-to and adhering to a well considered philosophy can help focus your creativity while avoiding unproductive tangents.
Over the years I've developed a philosophy about fly tying which I hope you'll find interesting as well as useful. In order of most important to least important, the ideal fly is:
Many great fly patterns meet only two or three of these criterion, but the closer you come to the ideal, the better. A fly pattern that is very effective, but at the same time, expensive, difficult to tie and fragile would not, in my opinion, be a great pattern, despite it's productivity.
Regarding the style of flies I design I have one overriding concern, whether it be a dry fly or streamer (and even nymphs and wets, though to a much lesser extent). Whenever possible, I like to see my fly! This is particularly important when fishing fast, broken surfaced freestoners, though less critical in glassy spring creek-like conditions. Though it is not necessary to see your fly at all times, most anglers rather enjoy a highly visible fly and I believe it results in more trout sighted as well as landed. Furthermore, it is my contention that there is almost always a way to make any given, and hard to see, fly pattern more visible without having a negative impact on it's effectiveness, and this is something I strive for in my designs and tying.