"The game features humankind for a reason. It is the most logical basis in an illogical game. From a design aspect it provides the sound groundwork. From a standpoint of creating the campaign milieu it provides the most readily usable assumptions. From a participation approach it is the only method, for all players are, after all is said and done, human, and it allows them the role with which most are most desirous and capable of identifying with. From all views then it is enough fantasy to assume a swords & sorcery cosmos, with impossible professions and make-believe magic. To adventure amongst the weird is fantasy enough without becoming that too! Consider also that each and every Dungeon Master worthy of that title is continually at work expanding his or her campaign milieu. The game is not merely a meaningless dungeon and an urban base around which is plopped the dreaded wilderness. Each of you must design a world, piece by piece, as if a jigsaw puzzle were being hand crafted, and each new section must fit perfectly the pattern of the other pieces. Faced with such a task all of us need all of the aid and assistance we can get. Without such help the sheer magnitude of the task would force most of us to throw up our hands in despair."
By having a basis to work from, and a well-developed body of work to draw upon, at least part of this task is handled for us. When history, folklore, myth, fable and fiction can be incorporated or used as reference for the campaign, the magnitude of the effort required is reduced by several degrees. Even actual sciences can be used - geography, chemistry, physics, and so forth. Alien viewpoints can be found, of course, but not in quantity (and often not in much quality either). Those works which do not feature mankind in a central role are uncommon. Those which do not deal with men at all are scarce indeed. To attempt to utilize any such bases as the central, let alone sole, theme for a campaign milieu is destined to be shallow, incomplete, and totally unsatisfying for all parties concerned unless the creator is a Renaissance Man and all-around universal genius with a decade or two to prepare the game and milieu. Even then, how can such an effort rival one which borrows from the talents of genius and imaginative thinking which come to us from literature?
-- AD&D Dungeon Master's Guide pg. 21
Up until recently, I would have said that I strongly disagree with Mr. Gygax on this point. I don't often play humans, and I tend to run a lot of games that feature non-humans prominently, even as majority populations. I have even brought up this section in discussions as an example of how many modern D&D settings have improved upon the original concepts layed down by Gygax and Arneson. I perhaps even bragged that I was one of those "Renaissance Man and all-around universal genius". However, when a group of players in my recent Dawn of Worlds game, decided that humans should not exist as playable characters and went out of their way to create events leading to that race's demise, it prompted a lot of discussions about the need for a relatable baseline in the world (even if that baseline is a minority or oppressed population).
At first I was willing to play along with the no-humans world. It even seemed rather fun and novel, when a dominant species ended up being described as "post-human, quasi-troglodyte, fish-people". That is, until I started trying to write adventures to be played in that world. The lack of a human baseline makes certain common events difficult to explain as being a threat.
Why should PCs be concerned about saving people from floods when 90% of the people can breathe water? Why should they care about crop shortages and famine when the people are insectivores? How can you save a kidnapped noble child when the youngest members of the ruling class are in their 40's? How do you tempt them with wealth when the only currency is salt? How do you frighten them with the risk of mutation and magical transformation when they already change shape at a whim?
This experiment has created what is certainly a very interesting world, with numerous possibilities, but the lack of a relatable human baseline does, it turns out, make something that is "destined to be shallow, incomplete, and totally unsatisfying".
So, my hat is off to you Mr. Gygax, you have once again proved your wisdom.