Interesting responses. My own instinct is to take quite a hard line on what qualifies as a true spey fly; on the other hand, as Craig points out, the art of fly tying needs to strike a balance between preserving the past in aspic and producing flies that will do the job for which they are intended - catching fish. Tradition should be acknowledged but not allowed to hamper creativity. Nevertheless, given the distinct stylistic differences between, say, spey, dee, simple strip wing and built wing flies, to say nothing of more modern styles of fly, I certainly don't think think that they can all be lumped together as speys and streamers, in the same way as we can acknowledge a number of quite different styles of dry fly, each with its individual characteristics.
In addition to the points made, I would suggest two further criteria. The wing, whatever it is made of, needs to have the classic spey profile. This means it not only must be set low to the body, but also should have a distinct humped or arched profile. Obviously this in easier to achieve with some materials rather than others; the classic bronze mallard lends itself well to it, but some feathers are altogether too straight in the fibre (and thus perhaps better suited to dee flies). It might also exclude hairwings, although I wonder whether an approximation of the classic spey profile might not be achieved by layering bunches of hair of different lengths. Incidentally, although the Orange Heron has clearly been regarded as a spey fly for many years, and shares most of the other defining characteristics, I think it is not easy to produce this classic profile using hackle tips, even if properly tented.
The other point I would add is that these flies should basically be quite unornamented. The point has been made that tails, where used at all, should be short. I would also add that butts, tags, veiling etc don't really belong on a traditional spey fly, although from a fisherman's point of view I confess that a slim tinsel tag can add something to the allure of a Lady Caroline, to my eyes, at least. Whether the fish appreciate it equally, I don't know!