Prior to starting out, my hopes for this book were not high. Literally, the first word of my notes was "why?"
The reason for my reservations is because the Ethereal Plane is a transit plane, and so far Planescape has been treating it as an afterthought. The Ethereal is between the Prime and the Inner Planes and the Astral is between the Prime and the Outer Planes, and you needed powerful spells to get use these routes, so it's a good thing Sigil has so many portals. You'd never actually go into them, because if you're in the Ethereal or the Astral, then by definition you're not where you want to be.
The book addresses this issue straight out the gate, saying on the first page: "The Ethereal contains everything material . . . a physical plane that literally is space, and all that space contains." Rick and Morty made the same observation, but meant it as a joke, presumably because it was so fatuous. Space exists between everything, so it could be said that those things are inside space, but space begins where the first thing ends and it ends where the next thing begins, so it's only space when there's nothing there.
The Ethereal plane surrounds all the worlds of the Prime Material plane (the book is coy about whether it exists between Spelljammer's crystal spheres, but by even bringing it up in the first place implies that Spelljammer is still canon, because what the setting needs is two completely redundant types of outer space) and it surrounds all the various Inner Planes, so if we're saying that it contains the things it surrounds, then this book should be about all the Prime worlds and also redundant with The Inner Planes . . .
And you know what, it almost pulls it off. It does what The Inner Planes should have done and is primarily an outer space supplement. It elides the main destinations, giving us no new Prime or Inner Planar locations, but luckily D&D lore has one more trick up its sleeve - demiplanes. Demiplanes are self-contained worlds that are mostly smaller than Prime Material worlds, but sometimes not. Why isn't Ravenloft a Prime Material world? Why aren't the demiplanes of Shadow, Electromagnetism, or Time full-fledged inner planes? Why is the demiplane of Nightmare considered a physical world and not a spiritual world like the Outer Planes? No one can say.
What makes A Guide to the Ethereal Plane such a great book is that none of that matters. You pass through the Ethereal Plane in order to get to some more interesting places, and the bulk of this book is actually about those places and they are generally pretty interesting. So, why the demiplane of Time? Why not? It's a place to go and now you can do time travel stories. Three of the book's chapters are mostly just collections of destinations and, ironically, it's the small destinations that get the most wordcount.
Probably just a byproduct of specificity. The demiplane of electromagnetism gets two paragraphs because it's basically an Inner Plane and thus all we need to know about it is the way it will kill the PCs (minor, but unending sparks on anything metallic). By contrast, The Demiplane That Lives is a character with complex motivations and a strange biology. It's like a giant cell, with continent-sized membranes folding in upon themselves, filled with "organ buds" and an oxygenated fluid that most mortals can safely breathe. Towards the center is the Visage Wall, from which it can form faces that will communicate with visitors, asking them a ton of questions as its child-like intelligence seeks to understand the universe. But don't lose patience with it, or else it will start digesting you, absorbing your memories in the process.
And though that may be the weirdest one, most of the other small demiplanes are similarly well-constructed. Crystal islands floating in gelatinous seas that have healing properties but will eventually trap your spirit. The plane that is like a hollow earth, but surrounding a black hole instead of a star, and where a ruined civilization slowly decays under a blood-red sky. "The City That Waits," which was once home to a bunch of reformed demon-worshippers, who were cursed by their former patron to fall into a deep slumber, its streets now stalked by The Vestige, a being made from their collective nightmares. It has an oddly elaborate canon history (the demon they worshipped was Orcus) and the weird, Gygaxian name of "Moil," but is not, as far as I can tell, a reference to an old-school module.
It's weird, we're two or three books away from the end of the line (A Guide to the Ethereal Plane is pre-Faction War and it's only because of a mix-up that I'm reading it out of order), but it's only now that it seems like the brakes are finally coming off. To quote the setup to the random demiplane creation table (that is inexplicably not in the original boxed set) -
Because demiplanes are such unique areas, DMs should feel free to let their creative impulses run wild. A realm of solid bone, animated weapons and armor, or intelligent weapons - it's all possible. These pocked dimensions are places where literally anything can happen . . .Turning the multiverse upside down is part of the charm of designing a pocket dimension.
Put this book in a time machine and send it back to 1993, with the "demi" and "pocket" struck out, because it gets to the very heart of what makes portal fantasy so compelling. It's so good and so obvious that I'm forced to wonder why it took so long to see print.
I suspect it's precisely because demiplanes are so unimportant that they're allowed to be weird. You don't have to commit to making one an entire world. It's just a demi-plane. Most of them are canonically created by an 8th-level spell, so much so that the group of dwarfs trying to build one by collecting chunks of stable protomatter are treated as a (mild) joke. Therefor, you can suggest in your random generation table that a world might be composed entirely of copper coins and act as a refuge for free-willed golems because there's a 143 in 144 chance that the world is no more than 100 miles across. It's the liberation that comes with being an afterthought. Clearly, someone at TSR asked the same "why" question as I did at the start of the post, and Bruce R Cordell answered "why not?"
A Guide to the Ethereal Plane's greatest strength, then, is that it is peak Planescape. Its biggest weakness is also that it is peak Planescape. This book is not a team player. It's a great source of planar adventures, but it is also an alternate model of planar travel that is largely redundant with Sigil and its portals. You can move from demiplane to demiplane by sailing on an enchanted ether-ship or by psychically manipulating the protomatter to propel you forward, but portals are thin on the ground, because if you had a portal, you would have no reason to enter the ethereal in the first place, you'd just skip straight past it to go directly where you want to be.
That's probably why the only portals mentioned here are in the Godsmen headquarters and providing water to a small town on a barren patch of rock too small to even be a demiplane. We've got a new cosmology now and we're not missing the Outer Planes at all! (That was, at most, 65% sarcastic).
Honestly, given the presentation in this book, you could almost do a pure ethereal campaign. The only real impediment is that entering or leaving the plane requires a level 7 spell (either Greater Etherealness or Teleport Without Error). That's because the ethereal also doubles as D&D's explanation for why ghosts can walk through walls. If you're close to a prime world, inner plane, or demiplane, you're in the "Border Ethereal" and there's a kind of mist that overlaps normal terrain. You can see the world, but you can't interact with it physically, nor it with you. Thus every ethereal travel power is also a top-tier spying and defensive power, and not something you can just hand out to PCs (until, apparently, 13th level, then it's fine for wizards to have it). It's a major impediment to the main campaign model - world hopping from one monster of the week story to another, but I figure it's got to be solvable. If you play with the Etherfarer kit, your special benefit is "an item that allows them entry onto the Ethereal at least 1/week, even if temporarily," which suggests that I'm not alone in perceiving this problem, but is also, strangely, a vague and incomplete solution.
I guess I would solve it by introducing a new way of breaching the Border Ethereal that was simultaneously conspicuous, inconvenient, and mundane. That way the stealth aspect of ethereal travel is mitigated, you can't flee from an ongoing adventure without laying some groundwork, and you don't need a high-level wizard to help you. What that would look like, I'll save as an exercise for when I actually run this campaign, but it's definitely something that would have made the book stronger.
My final impression of A Guide to the Ethereal is that it is an odd book that had no business being as good as it was and if you did ever want to do a Planescape reboot, you should steal from it liberally.
Ukss Contribution: There are so many great options, but what makes this job even harder than usual is that one of the chapters is all about ethereal monsters that assembles a greatest-hits collection of previously published creatures. Phase Spiders, Ebon Tigers, Thought Eaters - all your favorites are here. There are even some strong original creatures like Nathri (venomous goblin-like humanoids that are given full PC rules) or Memes (intelligent patterns that are constantly drawing in nearby matter, cycling it through themselves to create a sort of "body" and then ejecting it in a steady stream, basically like a standing wave in the ethereal plane).
I really like the Gk'Lok-Lok (except for the name). They're these skinny creatures of filigree metal that live on a crystal tree and like to trap the lost souls of dead heroes. Not for any sinister purpose, just to admire them for awhile before sending them off to their final destination, none the worse for wear.