For example, the new classes in the front of the book would make a cool addition to any OD&D/D&D/retroclone game. I particularly dig on Rob's Myrmidon of Set class. On first pass these guys look a little like an Egyptian themed anti-paladin type, but they are actually badasses for Law. It's a harsh, unforgiving Law, but these snake-worshipping sons of bitches are on our side! Very juicy. The magic-user subsection is chock full of alternatives to standard Vancian fire-and-forget spellcasting. And the cleric section is a good example of how to tailor that class to the religions of your specific campaign world. The thief section introduces the least offensive skill system I've seen in D&D. I still don't like skill systems in D&D, but I'd use Supp VI's if a player was keen on it. There's lots of other neat stuff in Supplement VI: an easy-as-pie ritual magic system, a theory of magic, new PCs races, new magic items. This is one of those books that is just full of stuff you can cherrypick for your own use.
While you can see some threads running through much of Supplement VI, I wouldn't say there's an overarching theme to the whole book except for the delightfully simple "Here's how my game works". Rob isn't trying to revolutionize the gaming world, he's inviting us into his much-beloved corner of it. I love DMs who do that. In that way, Supp VI is a callback to the hippy days of Arduin.
Right now you can get a PDF download from RPGnow for seven bucks, but a lulu print edition is forthcoming. In the interests of full disclosure I should note that Rob generously gave me an editing credit for looking over a draft of the first two sections of the book and a second credit because he used (and niftily expanded) my Hedge Mage rules. But the Majestic Wilderlands is Rob's baby and he should get all the kudos. I give Supplement VI: The Majestic Wilderlands a hearty recommendation.