This mostly shows up in the Prestige Classes. There's not a one of them that's just a fundamental build for the base class. Well, maybe the Frenzied Berserker (what if: a barbarian that raged), but even the Deepwood Sniper, the Shifter, and the Foe-Hunter do their shtick remarkably better than the base classes. The only one that's really a dud is the Exotic Weapon master, and that's mostly because "exotic weapons" aren't really a thing. I mean, they're a thing in the rules - weapons that are powerful and unique enough to merit charging a feat for proficiency - but that's not something that would have any meaning at all within the setting. "Oh, you're an expert at the blowgun, the whip, and using a bastard sword one-handed? Wow, what an . . . interesting life you must lead."
It's not just the extra design experience that makes this my favorite book in the series (so far). I also like that it has a strong theme. It's not like Sword and Fist, which put two classes together because they were both ostensibly skilled at fighting (one of the fundamental aspects of the game). Nor is it like Song and Silence, which put two classes together because they were both rogue sub-types in 2nd edition. This book is all about characters who thrive in the wilderness . . . even if the Barbarian sometimes felt shoehorned in.
Don't get me wrong, the primal Barbarian from 4th edition, with its elemental and shapeshifting rage powers, is one of my favorite classes in an edition filled with great classes, and I think you can trace that back to this book. Maybe not directly, but I figure that the decision to label the class a "wild" class, rather than a "martial" class must have put it in a conceptual space where the primal Barbarian was possible. It's just that, given the mechanics of the 3rd edition Barbarian, the game was really doing the class a disservice by making it a cultural practice rather than a personality.
"Warrior who fights with reckless disregard for their own personal safety" is a concept that transcends culture, or even genre. It's the Incredible Hulk. It's Jayne from Firefly. It's Viking berserkers. It's Kikuchiyo from Seven Samurai. In other words, a very appealing character type. You could easily do an "urban barbarian" and have it make more sense than an "urban ranger" (a character option included in this book, which I don't exactly dislike, except in the sense that it adds "determined tracker and manhunter" to a class that's already overburdened with too many archetypes - two-weapon fighter, archer, nature guy, semi-priest, violent racist - pick a lane . . . except maybe that last one).
However, as we saw with 2nd edition's Complete Barbarian's Handbook, the Barbarian class has the misfortune to sit smack dab in the center of D&D's worst quality: it's unexamined colonialism. Yeah, we get "savage" as a noun, though frankly the way this book uses it as an adjective (along with "primitive") is also pretty borderline. Barbarians are from hunter-gatherer type cultures (i.e. "barbarians") and they get Intuit Direction and Wilderness lore for the same reason they get illiteracy - they are assumed to come from a culture without advanced technology. They are "masters of the wild" by default. That's probably why their material here is relatively weaker than the Druid and Ranger focused stuff.
That being said, I think Masters of the Wild makes a pretty persuasive case that these books work better with three classes instead of two. It really only has room for one dry exploration of a niche rules topic: an excruciating discussion of the Ranger's favored enemy mechanic, in which the suggested advice is inadvertently a point-by-point breakdown of why it's a bad mechanic ("you might be tempted to choose undead as a favored enemy. Resist the temptation. All undead are immune to critical hits and mind-influencing effects . . . Thus, favored enemy bonuses are all but useless" - no, seriously, if you find yourself in a position where the nature guy can't have a burning hatred for necromantic abominations, you've written yourself into a corner and should start over). The organizations and general worldbuilding are missed, but they were really more of a DM thing anyways.
Overall, I'd say that Masters of the Wild really shows 3rd edition starting to come into its own. We see here the potential for the edition's mechanical complexity to support strong flavor choices, and the abundance of options really does feel more liberating when many of those options are objectively cool (Oozemaster Prestige Class, anyone?) I probably should have saved it for last, because it's going to be a tough act to follow.
Ukss Contribution: I've got a lot of great choices here. I'm tempted to go with the most off-the-wall pick - opposable weapons. They're magic weapons that can be wielded by creatures without human-like hands, thanks to incorporating their own built-in thumbs. I can't even imagine the spatial logistics for how this might work, and every time I try my brain drifts into the realm of cosmic horror ("This enchantment creates one or more thumblike projections on the weapon. These artificial 'thumbs' fold around the appropriate limb of the wielder to allow proper use.") Normally, this is exactly the sort of fantasy I like, but the final product is so niche I can't imagine anyone with both the ability and the motive to make them.
So I'm going with my second choice, the almost as whimsical Cloudwalkers spell. "You create gaseous pads of cloudstuff on the subjects' feet, allowing them to walk on the clouds." I love it when spells are allowed to drift into absurd cartoon logic.