So, I am pleased as punch so far with my printed copy of the Rules Cyclopedia.
A very few observations:
1. Magic Users get two first level spells at level one, keeping the Mentzer rule from his Basic Set. This is excellent, because "Read Magic" is required for any scrolls found while dungeon-ing.
2. Magic Users can, with the DM's permission, expand the weapon selection from daggers only to blowgun, bottles of flaming oil, bottles of holy water, staves, slings, and whips. A whip-wielding wizard is so very much D&D, or should be. I checked the Master Set rules, which first expanded the magic user weapons, and noticed that it did not include slings.
3. The artwork, which is widely panned, is actually not that bad. There are some very good pieces in there--a couple of guys hiding in a pond to escape orcs is one. Granted, there are some stinkers, too. Someone should do a poll whether the RC halfling or the 5e PHB halfling is worse.
4. Weapon Mastery seems very interesting; I might whip up some 4e-style "power cards" for players to use, though.
5. Dwarves can use polearms (at a penalty). Cool. It wasnt specified in Moldvay or Mentzer whether they could. It wasnt exactly prohibited, but the dwarf was supposed to be restricted to "normal size weapons".
6. I find it disappointing that a section giving examples of map symbols was not included. Also, I think there could have been a sample dungeon, either the Haunted Keep, or Castle Mistamere.
8. I was a little frustrated trying to find some specific rule that I knew I had seen in a prior reading.
9. The Paladin, Avenger, amd Knight are prestige classes for fighters who do not wish to ascend to the nobility. No ability restrictions, either, although a WIS score of 13 or higher is necessary for casting cleric spells.
10. Thieves. Ugh. This is something for all editions, really. I am not talking about the slow progression and the practical uselessness of thief skills at low levels, either. It's the idea that the class is a "thief". I agree with Matt Colville that the class is actually something like "dungeon crawler", because all the skills are useful for dungeon exploration. Lamentations of the Flame Princess calls them "Specialists", which is a step in the right direction. I plan on dispensing with the whole "thief" archetype, replacing it with "Adventurer" or "Dungeoneer" or something, and the "Thieves Guild" will become an "Explorers' Society" or "Adventurers' Club", a sort of semi-informal place to congregate with other Dungeoneers to trade stories, make connections, participate in training seminars, hear rumors, and the like. Nothing nefarious, but not necessarily well-regarded by those outside the craft. Name level dungeoneers can open their own clubs.