My biggest complaint about the Earthdawn Player's Guide stems from the thing I enjoy most about it. See, in Earthdawn, you play an Adept, a person who uses magic to perform phenomenal feats of skill. Warrior Adepts strike harder than a charging bull. Thief Adepts skulk through the shadows with preternatural grace. My absolute favorite is the Sky Raider talent that lets them negate their falling damage and jump from airships in a precision aerial assault. That's both a neat bit of world-building and a cool image on its own.
There are fifteen Adept "Disciplines" (ie character classes), covering a variety of heroic archetypes. And I like that a lot. My complaint is that a significant portion, 4 out of 15, of the Disciplines are spellcasters and taken together, they undermine the Adept system in ways I'm not sure it can recover from.
Don't get me wrong. Wizards and Elementalists and Illusionists and Nethermancers fit well into the Earthdawn setting, and it is both appropriate and wise to include them as character options. It's just, the portion of the book given over to spells and spellcasting is 95 pages. The portion of the book devoted to all of the other talents for all fifteen Disciplines (including those belonging to the four spellcasting Disciplines) is 64 pages. One quarter of the Disciplines gets one and half times more wordcount than the other three quarters combined.
It bugs me. On the one hand, the book is saying "magic permeates this world and is used in a variety of activities and fields, rather than being sequestered away in the dusty old tomes of specialists" and then it immediately walks that back by putting most of the flashiest special effects into the dusty old tomes of specialists.
It's not that spellcasters are necessarily more effective than other Disciplines. In fact, I'm fairly sure that given the xp sink that spells represent, nearly every Discipline is going to be better at its niche. It feels like a missed opportunity to give other fantasy archetypes a greater diversity of wondrous feats.
But that's a fairly minor complaint. Overall, I would call the Discipline system a success and my whining merely an aesthetic preference. The only thing that I'd call out as a flaw is the "step system" method of determining dice pools.
The way it works is that your attribute and talent ratings combine to give you a "step" and then various bonuses and penalties (such as equipment, magic, or the specific maneuver you're attempting) can raise or lower your step. Your final step determines what dice you roll, as indicated by a chart near the beginning of the book. At the low levels, it's fairly intuitive. Step 4 is 1d6. Step 5 is 1d8. Step 6 is 1d10. And so on. But then, once you start rolling multiple dice, it gets a little hard to follow. Step 28 is 1d20 + 1d12 + 2d8. Step 29 is 1d20 + 1d12 + 1d10 + 1d8. And step 30 is 2d20 + 2d6. There's a logic to it, but realistically, you're going to have to be checking that chart constantly.
I think you'd get used to it, though. And it would be worth it to do so. Earthdawn as a setting as a lot of great fantastic conceits that make it a one-of-a-kind experience. It is the only fantasy rpg I can think of that is explicitly post-apocalyptic. Most others have a vaguely sketched out disaster that ended a previous golden age, but in Earthdawn, that disaster is within living memory and the current society is the first and second generation descendants of people who huddled for safety in underground bunkers while ravenous Horrors scoured the surface of all life. There's even a location called Bartertown, which I have to assume is a deliberate homage.
Other things I like are Blood Elves, a group of elves whose magical protection against the Scourge failed and in desperation turned to blood magic to cause living thorns to grow inside their bodies, depriving the Horrors of the opportunity to derive sustenance from their suffering. Or the Sea of Death, which is an ocean of open lava, where specially shielded airships fly over it to harvest pure elemental fire. Or that trolls get around the fact that they are too big to ride horses by domesticating dinosaurs.
It's very cool, and honestly I wish there was more of it here in this book. The explicit setting chapter is less than 15 pages long, and much of the feel of the game has to come from examples, chapter quotes, and the odds and ends sprinkled throughout the book. I feel like 1st edition maybe did it better, but that was a long time ago, and I may be remembering the past with rose-colored glasses. (Sadly, I don't have any 1st edition Earthdawn books, probably my most regrettable collecting oversight).
UKSS Contribution - This is a tough one, but I'm going to have to go with sealed kaers. It's mentioned a couple of times throughout the book that not everyone has emerged from their magical bunkers yet. Even now, decades after the Scourge has ended. I think it would be cool to have a couple of those in the World of Ukss, even if the thing they're hiding from has to be changed.