From its first appearance in 1922 to the current wave of films today, 3D has always been hailed as a great technical advance which would bring the cinema closer to its future as an all-encompassing form of entertainment. This future, alas, has always remained just over the horizon, and the reason is plain to see: it has always required special, add-on technologies that have made films more expensive to produce, project and view. This has led to cost, which has led to its being seen as a premium entertainment, which has prevented it from becoming more widely used. Doubtless the current wave of 3D will fade, but in the meantime, it might be educational to take a look at Teleview, the very first 3D system for the cinema, as nearly all of the technological elements -- and all of the hurdles -- were there are the start, nearly ninety years ago.
Basically, there have always been two methods of achieving the effect of 3D -- one, as with Kinemacolor, was an active method using alternating frames of the film for left-eye and right-eye views; such systems then required either a polarizing filter (with the projected images also alternating in polarity) or a synchronized, electrical shutter for every viewer (this was the method of Teleview, and seen in the diagram of the viewer above). Oddly, this is not only the earliest, but the latest, system: 3D television similarly uses alternating frames, along with a special set of electronic glasses designed so that each eye sees only the frames made from "its" perspective (at $50 a pair, they're hardly cheap).
The other method, the passive one, is to project both left-eye and right-eye perspectives simultaneously, and use either red/blue or polarizing eyeglasses so that the overlapping images are "sorted out" by each eye. This has the advantage of cheap, disposable means of reception, but the disadvantage that the image on the screen will be poor to anyone without the eyewear. While we often associate this system and its red/blue glasses with the earlier heyday of 3D in the 1950's, polarizing glasses were in fact far more commonly used, primarily because such films did not have to be printed on colored stock, or use color at all.
Today, converting a modern multiplex cinema to 3D costs about $300,000 a screen -- which, at some larger houses, would mean several millions of dollars. The practice has therefore been to convert only a few screens, which means that any film released in 3D will be on fewer screens, and even with a premium will make less for both the studios and the exhibitors. The dwindling economic returns of such a thing, especially in the current recession, have caused some studios, such as Warner Brothers, to pull out of earlier commitments to making films, such as the last Harry Potter features, in 3D. The jury is still out on 3D TV, and my bet is that, before too long, we will once again associate 3D, that magnificent technology of the future, with the past.