I dispute the claim that "there was a lot of content there". There are a lot of words there - I counted them as 700-odd.
But there is not a lot of content there.
Being told that a monster is grey, hairless and with a thick hide (when I already knew, from its stats, that it is AC 5) is not significant content. How often has the issue of doppelganger body hair ever come up on a game? Or the thickness of its hide?
And how does it improve anyone's game to be told that the AC 5 is due to thick hide rather than (say) agility, or luck, or magic, or some combination thereof?
Pemerton, I'm still reading through this thread and I was intending to save my comments until the end, but I have to speak up now on a couple of scores:
(1) You're basically right that "impressive" is probably not the right word to describe 2nd edition Doppelganger lore. It's nothing like as good as OSR lore. It's impressive to me compared to the 5E MM's Doppelganger entry but it's not uncontrversially a masterpiece of impressiveness. If MaxPerson had used a milder word we might not be having a semantic argument right now over what impresses who.
(2) There's nothing contradictory about being "basically lazy" and yet also careful planners and diligent about things that have a high payoff. Actually that's an excellent description of
me. The classical description of a computer programmer: someone who is so fundamentally lazy he will spend three days writing a program to save himself five minutes, just so he never has to do that five-minute task ever again. Some people say you
have to be lazy to be a good programmer, though that's probably overstating things. It's easy to imagine a Doppelganger with the exact same mindset: work like crazy for three weeks to get yourself into a position where you'll have luxury for decades.
(3) Having a defined appearance for a doppelganger does enable certain stories. The first thing that comes to my mind is, "The King's best general has gone missing, and a hairless rubbery gray humanoid was found dead in the general's locked apartment. We think the Greys, whoever they are, must have kidnapped him, but he fought and managed to kill one of them, giving us our best clue! You're being paid to track them down and get him back!" Of course savvy players will jump right to the next phase of the adventure, "There was no kidnapping. It was a doppelganger all along--don't look for the general, try to figure out how long the doppelganger has been here and how much damage he's done, and who was paying him."
(It will turn out if players investigate that the Greys also "must" have poisoned the general, because his food has traces of poison, which provides yet more confirmation for the doppelganger theory and also raises troubling questions of its own.)
Of course we also know from doppelganger lore that the doppelganger was almost certainly being paid because being a top general is a high-stress position, and doppelgangers are basically lazy so wouldn't assume that kind of position merely for fun. It's not surprising that a powerful wizard might be involved, because hey, it's D&D, but at least we have explicit confirmation that doppelgangers are often associated with wizards for historical reasons, as opposed to, say, the drow. Hey, there's a powerful wizard on the border of the kingdom--he's supposed to be neutral in the Unhuman Wars but this doppelganger thing makes me think maybe he's not neutral, or maybe he might know someone in wizard circles who isn't. "Let's go check him out." Etc.
(3) It was not unheard-of in AD&D times for one PC to play under multiple DMs. I don't do that much nowadays but I understand that this still happens in 5E at Adventurers' League events, etc. To players in these campaigns it's genuinely useful to have explicit lore on the fact that doppelgangers have an appearance, and this is what it is, because otherwise different DMs would all make up different things and everyone would get confused.
(4) From a DMing angle, it warms my heart to be reminded that the 2nd edition MM includes information on Doppelganger psychology and motivations. All one tribe (so, high degree of social connectivity, probably a good grapevine, check--can make doppelgangers the rumormongers of the fantasy world); basically lazy but careful planners; not loners, operate in small bands; resolve conflicts by flight. Regardless of AC and combat abilities, it sounds like these guys aren't even supposed to be a combat challenge--they will rabbit early and often and fight only if cornered. All of these things tell me whether and in what manner the PCs are likely to come into conflict with the Doppelgangers in their midst, and how the Doppelgangers are likely to react. Contrast this with the 5E MM which gives about 25% as much information on doppelganger psychology ("work alone or in small groups, size unspecified"--well, I guess at least now I know there aren't any doppelganger nations out there, so it's at least half as useful as knowing that they operate in bands of 3-12; "might have daddy issues if you read between the lines", which is actually a cool bit of lore if you pick up on it) and perhaps a comparable amount of information on habits and tactics ("might has an imprisoned original around somewhere"; "seduces women and produces changelings").
When I grab a monster out of a book, its combat stats are a part of what I'm interested in, just in case it comes to a fight, but even more I want to know, "If there's a fight, what is the fight
about?" An MM entry which says, "Slaads will fight to the death for a pickle because they love the taste so much" tells me an easy way to inject a Slaad into my story, and a dramatic question for the players to engage with: "Can you stand to let a Slaad lick all the pickle juice off you after you fell into that vat? If not, can you
stop him?" 5E tends to assume that things will fight your PCs because the PCs are there, which is even sillier because 5E is set up so that fighting PCs is suicide, so why are all these things committing suicide-by-PC constantly? In this context it is even more important to have motivations for your monsters, and it's very helpful to have a motivation right there in the MM so that I've got a plot seed even if I have already used up all my good ideas for tonight and I literally just rolled this monster up on a random encounter table.
So the 2nd edition MM lore is, if not "impressive", at least "quite helpful".
(5) But I certainly wouldn't object to a MM with a few brilliantly-creative rumors per monster, like these:
http://hackslashmaster.blogspot.com/2013/02/on-ecology-of-doppelganger.html said:
They stole the source of true names and fled where only the damned go. They have returned, paying another price; their own visage. They imprinted the book on each of their souls, allowing them to take any form. You may have them do a task for you, but you must give your name to the book, allowing them the right to use your own face
When unconscious they revert back to their natural form
They are the liquid sea-formed soldiers of chaos, walking among the stone of the land to destroy law and all its stable forms. That war never ended
Doppelgangers are able to alter their voice to reproduce any sound
They evolved in a dungeon environment to take advantage of large groups of unorganized men. A party with more hirelings then it can keep its eye on at once is particularly prone to a doppelganger infestation
Some of these rumors (like the "unconscious reversion" one) become even more fun for the DM if they are false.