Fashion is as prominent in philately as it is in clothing or movies. Over the 150 years of our hobby many items that were once popular have lost collector appeal and some areas that were once neglected are now more popular. In the 1930s and 1940s precancel collecting rivaled all of the mainstream US specialties in popularity. Precancels are stamps that are sold already cancelled by the post office to indicate bulk rate mail usage. These stamps were enormously popular with thousands of different types existing based on city and denomination. By 1950, bulk rate payment was indicated by printing on the envelope and precancels were no longer needed. Their popularity quickly died (part of the reason too for the decline of the popularity of precancels is that they became increasingly difficult to find. Precancels were never accepted as legitimately used stamps by US collectors, but European collectors of US never understood the difference between postally used stamps and precancels so millions of precancels were shipped to Europe where they could be sold for more money). Perfins have a similar history. In the days before postage meters, companies would perforate the name or initials of the company on the mint stamps that they bought to discourage theft. Postage meters ended this practice and collecting of this area languished.
Foreign stamps too have seen numerous areas ebb in popularity over the years. All of the nineteenth century stamps of Great Britain were printed with plate numbers and it was very common fifty years ago, even in general collections, to find small plate number studies of these stamps. Only Great Britain specialists do this now. Used abroads and forerunners use to adorn foreign collections. They never do now. Traffic Light gutter pairs were the investment craze of the 1970's (on British Commonwealth issues). They are laughed at today.
There is a twofold goal for collectors when they decide what stamps they wish to collect. First, they need to find an area that is emotionally satisfying and fits their needs for a hobby. The specialty chosen should be neither too hard nor too easy, not too time consuming for the amount of hours the collector wishes to devote to his hobby, and not beyond his financial means (there are many wonderful specialties that can be almost completed at surprising modest prices). But important too is to pick an area that fits into the mainstream of philatelic practice and is not one of the fashionable fringes of the hobby that might not stand the test of time. This is not always as hard to do as it seems. Just avoid the ridiculous, such as the extreme grade conscious collecting that was dominating the hobby a few years ago (culminating in slabbed numerically graded common stamps that were pushed up to crazy prices ). Concentrate on the mainstream-the kinds of stamps and quality that has attracted collectors for over a century and you are sure to have philatelic property that is desirable when you go to sell.