[Sorry people, I know the blog is slowing down quite a bit. I have in preparation a good bunch of posts on LotR (both novel and movies) and quite a few on Fate and GURPS. In the meanwhile, a little old D&D.]
Historically, D&D manuals seem to have suffered from bad writing. I am not talking content here: of course, one could have countless bones to pick with this or that D&D edition, or even with all of them. I mean the plan of the work, and its style. I remember lending all of my AD&D2e manuals as soon as I stopped DMing (and never recovering them to this day), precisely because they made such a poor reading: having them for reference when you played was one thing, but otherwise they seemed a waste of space.
The Rules Cyclopedia, on the other hand, has none of these flaws. It is concise, well organized, and lively written. It does not indulge in preliminaries, nor it displays self-importance. There was no room for such things: RC was born to be the first and only comprehensive one-volume D&D rulebook. It includes everything one needs to run not only an adventure, but in fact a campaign that could last years or even decades. Of course, the crunch was the same as the BECMI boxed sets, so many things were simplified with respect to AD&D or later D&D editions: fewer classes, races-as-classes, fewer spells, fewer monsters and so on. And yet, it also included much that would never been included in core rulebooks again: rules for acquiring and governing a domain, rules for mass combat and sieges, not to mention the (however sketchy) guidelines for characters to attain immortality. So, while RC had everything somewhat simpler, it also has the grandest scale ever in forty years of D&D manuals.
And, to be precise, not everything was actually simpler than in AD&D2e: the weapon mastery rules (presented as optional) are actually much more detailed than the weapon proficencies in AD&D2e. Whether this is for better or worse, it is up to the reader to decide. Suffice it to say that while the rules complicate the (extremely, almost irritatingly) elementary nature of BECMI combat, they have at least two strong points. First, they greatly increase the offensive capacity of PCs: while this might posit some problems at medium to high levels, it is a welcome change at the lowest levels (PCs have now better chances to survive encounters with low level monsters, and we know that nobody likes to be stabbed to death by a kobold). Think of a poor first level magic user: now, if he spends both his weapon mastery slots on the staff, he becomes a skilled staff fighter, who can attack and defend effectively and inflict considerable damage. Second, it makes it possible to differentiate greatly between individual PCs, which is precious in a game that has always had the tendency to look at PCs as stock members of the respective classes. Two warriors can be finely differentiated not only in terms of their “liking one weapon over another”, but of having entirely different tactical roles and special features: in this sense, RC does not pale in comparison to D&D3.X. Even at first level, you can now create heavy cavalrymen, crossbowmen, archers, skilled halberdsmen or whatever you feel like. Not that the system is without its quirks: the very smart, and valuable, intuition that experts of a weapon are more effective against some opponents than against others ends up giving counterintuitive results, since users of missile weapons have been conflated with monsters with natural weapons (i.e., a swordsman has the same bonuses when fighting an archer as when fighting a dragon, but better bonuses when fighting another swordsman). Does it sound right? I am not sure.
Since we are very much talking Old(-ish) School here, I will go for something nerdy now. RC (like BECMI before it) has, as far as I know, by far the most powerful version of the Meteor Swarm spell: it inflicts an astronomical 32d6 damage on impact plus 32d6 of blast damage, against which a saving throw for half damage is allowed. A high level magic user has thus the potential to inflict up to 384 damage, which would be enough to wipe out the vast majority of opponents in one blow. In Pathfinder, for comparison, Meteor Swarm does not exceed a maximum of 192 damage. One might say that the RC version is “unbalanced” but hey, when DMs let 21st level wizards in their game world, what did they expect…?
[EDIT – 28/06/2016: I was wrong on Weapon Mastery rules. First-level characters cannot allocate more than one mastery slot to the same weapon, so a magic-user would have to wait to do her kung-fu. By the way, the RC as written is not super clear about this, unlike Dark Dungeons, which I strongly recommend for other reasons as well.]