Original Edition Dungeons & Dragons was a compact game with an open matrix, making it easy to add modules of rules into the system without breaking game play. This calls to mind the old saw about "that isn't a bug, it's a feature!" What are sometimes seen as errors of omission were often intentional, the base assumption being the referee would of course write his own sets of rules add-ons to fill in the blanks.
Pantheons: specific deities were absent from OD&D. There were ambiguous references to "help from 'above'" and "the 'faithful'" in the description of the Cleric. Furthermore, references were made to the Cleric serving either Law or Chaos with no further information given. The Magic-User spell Contact Higher Plane makes references to creatures inhabiting "higher planes of existence" without directly referencing deities. Similarly, the Cleric spell Commune talks about powers "above." None of the following words, including their plural forms, appear anywhere in the 3 booklets of OD&D: god, higher power, deity, or divine. Similarly, no infernal creatures (demons or devils) appear in the rules. On a side note: the Balrog appeared in the early edition of the rules but expunged after a threatened lawsuit from the Tolkien estate. This monster would reappear in a later rules supplement as a Type VI Demon. Curiously, the name "Balrog" is directly referenced in that work, too.
Coins: the rules list copper, silver, and gold coins. Electrum (a gold and silver alloy) and platinum coins are listed as "optional" and electrum is suggested be either half or twice the value of gold pieces. In the equipment lists, however, everything is priced in terms of gold coins.
Armor: you had the choice of leather, chain, or plate. Field plate or full plate, what most non-gamers envision when you say the word "armor" doesn't exist in the D&D milieu. Leather armor is never explained and the likely assumption war gamers would understand this to be cuir bolloi, leather shaped then boiled in oil or tallow. There was no jack, scale, banded, ring mail, or any other historical types appearing in later editions. The rules contained no references to zero or negative Armor Class, the toughest monster listed was AC 2. Magic armor subtracted its bonus from the "to hit" roll of the attacker, meaning the target number to hit either mundane or magical armor would remain the same. The rules also state that either the magical bonus of the armor or the shield apply, 2/3 or 1/3 of the time respectively, but not both.
Unarmed Combat: no rules for wrestling, brawling, or the various unarmed fighting arts such as karate or tae kwon do. A curious omission, though I feel the combat system lends itself well to running unarmed combat and brawling. It's worked well for me, anyway.
Critical Hits & Misses: to the best of my admittedly faulty recall? With one exception, the first official critical hit rule appeared in the 1st Edition AD&D Unearthed Arcana supplemental rulebook. The exception appears in Book III: The Underworld & Wilderness Adventures in the section dealing with aerial combat. This system uses hit locations for flying mounts, making the presence of a critical hit logical. Taking out the wing or head of an airborne creature would, after all, cause a rather precipitous return to earth! In my experience, which was rather limited with regard to games outside of [small town in Texas], critical hits were not popular at all though that certainly doesn't seem to be the case any longer. For myself, I don't care for them.
Variable Weapon Damage, Weapon Versus Armor Class Adjustments, Weapon Speed Factors: presumably it was felt Chainmail addressed these issues for gamers wanting these factors coming into play when conducting combat. If true, that line of reasoning was short lived, ending when Supplement I: Greyhawk was released.
Segments: as in Turns/Rounds/Segments. By implication a combat round was a minute long and not divided into smaller units.