DEITIES & DEMIGODS is an indispensable part of the whole of AD&D. Do not fall into the error of regarding it as a supplement. It is integral to Dungeon Mastering a true AD&D campaign. Experienced players will immediately concur with this evaluation, for they already know how important alignment is, how necessary the deity is to the cleric, and how interaction of the various alignments depends upon the entities which lead them. Those readers not well-grounded in ongoing campaigns must take my word for all this, although they will soon discover for themselves how crucial the deities of the campaign milieu are.Looking back on it now, I can see so many problematic statements in the few lines of the foreword quoted above, but, back then, Gygax's words were law and if he said the DDG was "integral to Dungeon Mastering a true AD&D campaign," then it was so. Now, though, I see things somewhat differently. A "true AD&D campaign?" What do those words even mean? That it's impossible to play the game without the information contained in Deities & Demigods? Or is it that Gygax believes the notion of gods to be a foundational principle of the game?
Either position is, in retrospect, somewhat questionable. Not only is it possible to play even AD&D without recourse to the DDG, it's possible to play the game without gods at all. Moreover, the particular approach to the divine adopted in Deities & Demigods is by no means the only one. I've noted before that early gaming often evinced a rather different approach, but, even if one adopts a polytheistic model, there are other ways to portray the gods than as, for all intents and purposes, extraordinarily powerful "monsters."
Deities & Demigods is, in many respects, just another Monster Manual, filled with pages and pages of statistics, such as armor class, hit points, and attack forms. Despite the claims of the foreword, the book did little to impress upon me the importance of gods or their worship in AD&D, as the book provides only the most cursory information on the role of religion in the game, instead focusing on extending ability score modifier tables and elucidating the powers of gods of various ranks.
If I were to point out a single, fundamental flaw in the way Dungeons & Dragons has developed over the years, I'd say that it was its relentless drive to quantify everything. It's a flaw Deities & Demigods demonstrates time and again. A book detailing various historical and literary pantheons, along with information on including them in a campaign, is a great idea. But why include game stats for the gods at all? Why tell us that Ra has 400 hit points or that Mjolnir deals 10d10 damage? How does this information accord with Gygax's claims that the DDG lays out "how crucial the deities of the campaign milieu are?" In my experience, such details only fueled foolish arguments over which god was most powerful, not to mention dreams of marching on Mount Olympus to slay Zeus. What is so "integral" to the game about this?
Truth be told, I don't hate Deities & Demigods. I simply don't see it as anything more than a minor volume in the AD&D library, less significant even than Unearthed Arcana, which, for all its manifest faults, contained more unambiguously useful information to the game. I also think it contributed to the power creep to which D&D has always been prone, as well as being part of a reductionist "demystifying" approach to magic and the divine. As a younger person, I enjoyed reading the book, since it included gods and mythologies with which I wasn't familiar. I didn't get much direct use out of it, however, since I'd created my own religions and didn't see any need to provide stats for divine beings. That's an opinion I still share, even though it runs counter to the trajectory the game followed and that seems to have appealed to many of its players.