Every couple of years 'pulp' becomes the popular genre in the gaming community, and then is cycled out of favor to be replaced by something else (I think super heroes is the current popular genre). Now, some may call me an elitist or a snob, but I am a firm believer in using literary genre labels to describe RPGs of differing genres. To this end, I am afraid to tell you gentle readers that there is no such thing as a 'pulp' genre. Still, the tag seems to be used by nearly everyone in the industry to describe any game set in the 1920's and '30's. In a sense, it has become shorthand for high action adventure, often with some nod to a historical period, set before WWII yet after WWI. Sometimes the decades are expanded to include the latter part of the Victorian period, and often conflated with both that era and the following Edwardian era.
So, what does pulp actually mean? In short, it is a type of paper, a low grade and thus cheap paper that was used for popular fiction magazines from the 1890's to the 1950's. These magazines were themselves often genre specific; there were detective pulps, western pulps, horror pulps, fantasy and science fiction pulps, even romance or true crime pulps. These magazines were aimed at the working classes amongst whom there was a growing literacy rate, and prior to the advent of radio and TV, provided a much-needed form of entertainment. They were not something that 'proper' people read (though they did, like most things 'proper' people don’t do). The quality of the writing was generally poor, the illustrations pushed the limits of the decency laws of the time, and the stories themselves were lurid, sensationalist, often violent, nearly always fairly far-fetched, and in general considered at the time to be of little literary value. However, it is out of these pulps that some of the greatest writers of the age came, most notably Bradbury, Chandler, Heinlein, Howard, L'Amour, Leiber, London, Lovecraft, Wells, and even Vance. Notice any names on that list that had a large and positive influence on fantasy, sci-fi, horror, and RPG's? Yes, the literary critics of the day were shortsighted in their view of the pulps, neglecting to see that amongst the rough there were pearls. We should all learn a lesson, especially those of us who are elitists, that being a snob is the antithesis of elitism, for we must always strive for and seek the best, no matter what rags it may wear.
Thus, pulp is not a true genre, but a style of publishing and writing. How then is it a genre of RPG's? Ignorance would be my best guess, but partially through the means by which a language grows and changes. It is quite normal for words to take on different meanings, though it can be very annoying when one is speaking of technical terms, and those who use pulp as a means of defining a genre should not be chastised for it (at least not too much or too frequently).
A pulp game should be high on the action, short on the plot, and thoroughly done for the fun of it. It should tend towards sensationalist and over the top behavior by the PCs and NPCs, and most of all should keep the action flowing. Character development should follow a relatively shallow track, we're not doing Masterpiece Theatre here, and emotionally responses should be played up almost to ridiculous heights. The Rule of Cool needs to be in effect, and a large amount of suspension of disbelief on the part of all involved is not just expected, but required. In this way you are holding true to the pulp magazines of yore, and to the style they put forth. Sex was often a selling point of the pulps, at least on the magazine covers, and although your game needn't be explicit, it should have some titillation. Fur and chainmail bikinis are optional, but both genders should receive a fair amount of 'cake' if you know what I mean. Why, there was even a 'caged heat' genre aimed at heterosexuals and homosexuals of both sexes (though it should be noted that these and other highly sexual pulps were rare and sold under the counter).
By treating pulp as a style and not a genre, you can then apply it to any genre you desire, thus emulating the magazines (and the crappy paper) that yielded the name. Considering that the pulps were the birthplace of many of the genres we now enjoy, from Sword and Sorcery to Super Heroes, this is not just appropriate, but nearly a requirement. Looking at gaming this way, I can safely say my current Barbarians of Lemuria game is a pulp fantasy (and made more so by the introduction of the PC named Sexy Fat Man). Many of the games I have ran in the past are pulp styled, and that's fine, it’s a style I love and love to see more of. It's just not a genre per se.