Random Roll: DMG, p. 21

Original Dungeons & Dragons famously opened the door to monster player characters, boldly stating "there is no reason that players cannot be allowed to play as virtually anything." On page 21 of the Dungeon Masters Guide, Gary Gygax all but closes that door and, in doing so, offers a very clear statement on his belief that "ADVANCED D&D is unquestionably 'humanocentric'." That's a perspective with which I strongly agree, though I suspect it's quite controversial these days (and, to be fair, was probably controversial in some quarters even in 1979). Let's take a look at what he says.

Gygax begins his examination of the topic of "the monster as a player character" by imputing an ignoble motive to any plater with "a strong desire to operate as a monster." He suggests that such a player does this "principally because the player sees the desire monster character as superior to his or her peers and likely to provide a dominant role for him or her in the campaign." This seems like the wrong foot on which to begin this discussion and one that makes it so much easier to dismiss everything he says afterwards. It's a common flaw in Gygax's authorial style, unfortunately.

From there, Gygax makes a subtle elision. The section is ostensibly about allowing monsters as player characters, but he quite quickly moves from monsters to demihumans and humanoids. About the latter, he says that they occupy "various orbits around the sun of humanity." He elaborates in a way that comports with my own thinking: 

Men are the worst monsters, particularly high level characters such as clerics, fighters, and magic-users – whether singly, in small groups, or in large companies … there is a point where the well-equipped, high-level party of adventurers can challenge a demon prince, an arch-devil, or a demi-god. While there might well be some near or part humans with the group so doing, it is certain that the leaders will be human. In co-operation men bring ruin upon monsterdom, for they have no upper limits as to level or acquired power from spells or items.

Gygax goes on to say that humanocentrism simply makes sense.

The game features humankind for a reason. It is the most logical basis in an illogical game. From a design perspective 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!

For Gygax, this seems to be the crux of it. He saw a need for grounding even in fantasy and believed that humanocentrism was the simplest means of providing that grounding. It's an unusual take on the question, though not wholly without precedent. Gygax seems to have felt that playing a non-human was exceedingly difficult and, in fact, an impediment to approaching fantasy. M.A.R. Barker believed something akin to this, in as much as he felt that very people could play one of a non-human character on Tékumel (which, to be fair, is plausible, given how alien many of them are). 

Gygax further comments that there are very few models for non-human characters to use as inspiration..

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 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?

I doubt many gamers today would be convinced by this line of thought. Even I, who strongly prefers humanocentric fantasy, find it a weak line of argument. I think Gygax's larger point might have been better served if he'd simply acknowledged his preference for humanocentrism and then marshaled evidence, drawn on his own experiences as both a player and a referee, of why humanocentrism led him to prefer human-centered fantasy. 

The remainder of the section deals with the rules-related challenges of allowing monsters as player characters. His concerns seem to be about "game domination" (to use Gygax's phrase), which echoes what he said at the start of this section. If that's his primary concern, I think it's a more understandable one, even to those who are temperamentally inclined to freely allow the playing of non-humans. Like so much in the Dungeon Masters Guide, there's a lot to chew on here, though it's couched in ways that muddle his legitimate points.