This got me thinking about what D&D would be like if instead of the original booklets and Holmes rules coming before the Moldvay and Cook books and the numerous versions after, the Moldvay and Cook books were it. An alternate universe where 1981 was the first year where fantasy roleplaying was introduced to the world in the form of the B/X box sets and, since they are perfect :), no other version or form of roleplaying game was ever published.
How would D&D play if you only had the two boxsets (the Basic and Expert rulebooks, the Keep on the Borderlands and Isle of Dread) and had no experience of 1974 D&D, AD&D or 2E AD&D, or any other forms of roleplaying.
1. Race as class would be a given. This would give the game a very strong focus are archetypes.
2. The implied setting would be different.
2a. Alignment - Three basic ways of life would guide the actions of everyone in the world (lawful, neutral, chaotic).
2b. Alignment Languages would be a definite part of the setting.
2c. Low magic - low power. Especially if you take the conservative view on spellbooks. Fighters would remain relevant at higher levels. Less powerful magic items.
2d. Deities are beyond the scope of monster stats.
2e. Dungeons and wilderness would be populated with NPCs, humanoids, giant animals, dinosaurs, and mythical monsters. It would be missing many of the "iconic" D&D monsters such as beholders, liches, demons, etc.
3. Dungeons would be smaller and scenario specific. I believe that relying on the instructions in the basic rulebook and the examples of the Haunted Keep and the Keep on the Borderlands would give a different feel for dungeon adventures.
3a. The design process for creating a dungeon is outlined as a very systematic process in the basic rulebook. Each dungeon would be designed for a specific scenario or quest, would include one or a few specific monsters and the rest would be randomly generated.
3b. Lots of characters would die facing the forces of chaos in the dungeons.
4. You can never trust NPCs as they may be agents of chaos.
5. Wilderness would focus on small scale and scope - valleys, islands, a barony, etc. and would be for exploration and to follow treasure maps.
5a. The wilderness design process focuses on a micro to macro process. Not far reaching campaign worlds.
5b. The base town is for healing, magic, rumours, retainers, and selling (read fencing) treasures found.
6. High level PCs and NPCs hang out in their strongholds.
7. Lots of sailing around in boats.
8. More structured game play. The Basic and Expert rulebooks are highly formulaic and following it from game session to game session gives a campaign a very distinct feel. It would be quest focused (even if many of the quests are just the PCs trying to become rich) and episodic. Without other roleplaying games to draw from, the mechanical processes given in the rulebooks for everything from moving in a dungeon, combat rounds, wilderness exploration, etc. give everything a very codified, concise, and almost board-gamey feel. And this is not a complaint - it is how I often try to run it anyway.