I'm often rather harsh about the D&D rules written in the mid-80s by Frank Mentzer. Part of it is because they're products of a decadent era at TSR, when Sutherland and Trampier gave way to Elmore and Parkinson and when mass market appeal drove most design decisions. And the Mentzer rules are nothing if not mass market. Even moreso than Moldvay/Cook, the Mentzer rules were written for people unfamiliar not just with roleplaying games but also with the entire culture out of which D&D sprang. They're the first truly "post-D&D D&D" and it's very hard for me to look beyond that much of the time.
That's a shame, because, for all their concessions to mainstream acceptance, there's some good stuff to be found in these boxed sets, particularly Set 3 -- the Companion Rules -- from which I stole lots of ideas for my AD&D campaigns. For me, Set 3 was the fufillment of a promise on which D&D had never quite managed to make good -- to provide rules and guidelines for the endgame of a character's adventuring career. The Conpanion Rules gave us not only domain management rules, but also a mass combat system. Together, these two systems addressed issues D&D had had ever since 1974: what do characters do when the reach high levels of experience?
The domain management and mass combat systems in the Companion Rules are simple, straightforward, and abstract. They depend very heavily on referee adjudication and there's a high degree of randomness to both of them that might offend some sensibilities. Quibbles aside, they're both quite old school in their approach and I like that. I played the heck out of both of them back in the day and eventually generalized the domain management system to cover random campaign events and the fate of entire nations rather than the single-hex baronies for which they were originally written. I also used the mass combat rules to wage wars between nations in my fantasy setting. Together, they gave that setting a healthy dose of unpredictability that gave it an air of "realism" it might otherwise have lacked. For example, a civil war broke out in the major good nation of the world and that sparked innumerable adventures, not to mention far-reaching consequences that I doubt I'd have chosen had I instead plotted out its future history myself. This is excellent stuff.
Another aspect of the Companion Rules that I liked a lot, even though I never incorporated them into my AD&D game, was the way Mentzer introduced paladins, druids, and other such classes into the game. They were kind of proto-prestige classes but far more elegantly implemented and far more grounded in the history of D&D. For example, I see in the druid an echo of the notion that OD&D clerics must side either for Law or Chaos once they reach 7th level or they cannot advance any further. Likewise, the paladin offers up a viable interpretation of the line that Lawful fighting men "may opt to become paladins." I've often considered reworking many AD&D sub-classes into a similar kind of scheme and may yet do so one day.
I'll admit that I'm not at all fond of the Master Rules, let alone Immortals, both of which strike me as huge missteps compared to the Companion Rules. Those final two boxed sets are the result of a corporate completist mentality rather than as answers to questions most D&D players had at the time. It certainly doesn't help that I think those sets also highlight the limits of D&D and not in a positive way. There is only so far that the game can go before it loses its identity and the heights of power detailed in the Master and Immortals rules are several steps too far in my opinion.
I'll continue to quibble about the Mentzer rules, because there's much to quibble about, but it's quibbling "within the family," so to speak. Frank's old school credentials are not in doubt nor is his grasp of the history of the game. I am, as I hope this post reveals, very fond of his Companion Rules set and believe it's the only real treatment of D&D's original endgame ever written. I'm not blind to the virtues of later rules sets, but I do have very specific criteria for what I like and what I'll consider using. That means I can be convinced that I'm off base when it comes to my criticisms of this or that. In the case of the Companion set, I need no convincing.