Important Disclaimer - 1st edition Advanced Dungeons & Dragons is a masterpiece. It is still one of my favorite RPGs and is one of the most important games ever published. Because of my love for the game, it would be too easy for me to come up with 101 things I like about it so I am challenging myself by trying to come up with that many complaints about it. This does not reflect any animosity toward the system, rather the opposite.
#1 Alignment Languages
I might as well start with the lowest hanging fruit, an by low hanging I mean so low hanging that it takes a shovel to pick it. Alignment languages are described as being "the special set of signs, signals, gestures, and words which intelligent creatures use to inform other intelligent creatures of the same alignment of their fellowship and common ethos." (DMG page 24). It is described as being "a handy game tool [. . .] not unjustifiable in real terms." (Ibid.)
Even the two examples given in the DMG, thieves' cant and the Latin of the medieval Catholic Church, are highly flawed examples of secret languages that might be similar to alignment languages. First there is the obvious problem that these real world examples were connected to occupations and not alignment giving them logical ways to be learned (e.g. during apprenticeship or initiation). No one has ever in fantasy or real life apprenticed to be Chaotic Evil. And, of course, these real world languages had no alignment restrictions. If a priest had a change of heart and stopped being good, he would not suddenly forget Latin. Also neither language, especially Latin, ever kept completely secret.
With no comparable real world examples, we must try to come up with a reasonable explanation of how such languages would be acquired. Parents could not teach their children because children often grow up to have a completely different ethos than their parents, who may not themselves be perfectly aligned with one another. Apprenticing for an alignment or being initiated into one might make for an musing short story, but is too absurd for a game or novel (the willing suspension of disbelief on goes so far.) This leaves only the "will of the gods" or some other other deus ex machinato save this hopelessly irrational concept. Unfortunately, there is no reason for gods to magically intervene and cause every person of a given alignment to speak that alignment language. The languages' uses are limited at best and assassins ability to speak different alignment languages (PHB page 29) undermines even them. Why would deities go to the effort of instilling this language upon some, but not all, members of an alignment for an alignment that can't be spoken in public without repercussions nor used elsewhere unless one is already certain of the listener's alignment? (DMG page 24). And the DMG strongly implies that this is not how it is acquired (ibid.).
So without any logical reason for alignment languages existing in a game would, there must be some indispensable game function such as balance for them, right? Unfortunately not. The most obvious uses might be to test the alignment of a possible recruit or to give commands in combat. The first is allowed, but the fact that assassins can speak alignment tongues makes it untrustworthy and the latter would not work because the vocabulary of alignment languages are limited to "the ethos of the alignment in general" (ibid.).
Even if they were useful and had a logical way of existing, I would still not care for them. They take something that should be abstract and subject to debate and place a layer of absolutism on it. Players should know their characters' alignments but he characters probably should not. Even paladins should likely consider themselves to be "good" and not say "lawful good." It's just that they consider the lawful good ethos to be pure good and any deviation toward either chaos or evil to be less good at best.
So let me reemphasize how much love first edition and respect its creator before positing that alignment languages are not just one of the worst ideas of any edition of D&D, not just one of the worst RPG ideas not in F.A.T.A.L., but easily one of the top 1% of bad ideas in imaginative literature.