When I first started reading this book, I was afraid it would be The Wilderness Survival Guide all over again. The first half is filled with dry, overly-fiddly rules for things like climbing, navigating a boat through underground waters, and mining for minerals.
Part of me is tempted to just run with it. Say, "fuck heroic fantasy adventure, this game is about running a mine now." I like it when video games lose the thread that way (and may be the only person on Earth who approves of Mass Effect 2's scanning minigame), but it didn't work in the wilderness book, and it doesn't work here. The particulars of what the tedious rules actually are are not very fun to engage with. (This is where I suggest using Chuubo's pastoral genre for games about mineral exploration and running a business).
However, Dungeoneer's Survival Guide is largely redeemed by its second half. It details an underground setting filled with various elemental creatures, cthonic monsters, and exiles from the surface.
The lands of Deepearth would work pretty well for a series of adventures, but it's an oddly conservative type of fantasy. I suppose it makes sense, given how early in rpg history it is, but it feels off when they talk about "live caves" and "dead caves" and you realize they mean nothing more than the presence or absence of water. Or when you've got a wide array of fantasy races, but they mostly look human and do human-type things. And when even your unspeakable abominations from before the rise of man still manage to live in cities and care about some rather prosaic forms of treasure.
It might just be an artifact of being a short subsection of an already pretty short book. The DM's section does have to split its wordcount between Deepearth and a surprisingly long diversion into to generic DMing advice.
And this is really basic stuff, not connected to dungeoneering at all, except on those intermittent occasions when the author seems to remember that this is a specialist book. We're talking about things like player motivations, campaign structure, narrative vs sandbox play. All of the traditional DMing advice we've come to expect from the D&D Dungeon Master's Guides.
Except, inexplicably, it's here. In a book that's supposed to be about underground adventuring. I can't explain it, except perhaps with the theory that people back then were still figuring out how this whole "roleplaying game" thing was supposed to work, so they just did whatever seemed like a good idea at the time.
In any event, I may have to track down a later, more sophisticated version of the Underdark, one that perhaps understood that giant, underground mushroom forests inhabited by a race of sapient fungus men, was something interesting enough to warrant more than a couple of perfunctory paragraphs.
UKSS Contribution: I'm going to go off the beaten path here a little. There were some cool setting details, like magma-men who were only incidentally dangerous, but at heart playful and silly creatures that enjoyed climbing up high cliffs and then diving into pools of lava down below. Or, you know, creepy mythos-esque creatures like Illithids and Aboleths.
However, in the spirit of the book being entirely too mundane for its premise, I'm going to choose something that could exist in the real world, but which the rules strangely decide to focus on in a way that implies that it happens all the time in D&D world - naturally occurring lodestone deposits that generate magnetic fields so powerful they pose a danger to adventurers wearing metal armor.
I can't find evidence that there was ever a real lodestone strong enough to yank an adult man off his feet at 30 feet away, but in fantasy geology, anything's possible. There's probably some occult explanation for why this phenomenon occurs, but it isn't necessarily more occult than the explanations behind most other features of the terrain.