I think a big portion of the uniform fetish is the idea that wearing the uniform is, in effect, the trading away a part of your identity in exchange for becoming a symbol of something else.
Why do policemen, firemen, doctors, lawyers (don't tell me suits aren't uniforms), even waitresses all wear the same thing? Obviously, so the person wearing the attire will be recognizable as filling some role. But this has the counter-effect of making you less recognizable as yourself. The concepts of exchange of identity and power each have their sexual aspect.
Not as obvious, when people put on hats, uniforms and masks (especially masks, see mask work), they lose some degree of their inhibitions. (There is a particular Bugs Bunny cartoon that illustrates this in an exaggerated way. It's fun to watch Bugs chase around a wisecracking Elmer Fudd for a change.) Wearing something that obscures your identity enables you to do things that would be considered upsetting, provocative, and even reprehensible, both to other people and yourself.
Looked at one way, everyone is nothing more or less than a person, with no special powers or abilities. Why should I stop driving just because you tell me to? Because some organization that calls itself a government hired you as a police officer, or because you look like someone who should be obeyed? The closer you look at these encounters with authority intellectually, the less weight they hold ("How many years of college have you had, Mister Policeman?"), so they all make use of some sort of uniform system, trading a portion of an individual's humanity in exchange for the ability to boss people around. Why do you think executioners in the United States wear those hoods, even now? I'd say it's not for the sake of the prisoner or the observers, but so the person pulling the switch will be able to permit himself to carry out an act which, let's face it, is difficult to explain away as being in your job description.
This is the darker aspect of uniform attire, but it can have a powerful positive aspect as well. In some cases one wants to trade away his humanity for his own purposes, and one of the best places to do this is in the bedroom. Being human is a complex set of moral imperatives and inhibitions, but if you can look in a mirror and see something a little more, or less, than human looking back, you can avoid some of each, and allow yourself to participate in acts you'd never ordinarily consider, boss someone around in an authoritative manner, or allow yourself to be bossed. So long as no one is hurt, and you go back to your ordinary selves afterwards, what's the harm?
My very first, and probably very last, sexual writeup