12. It is unapologetically gameist in nature - doors in dungeons are stuck closed. You need to roll to force them open. Why? Because the rulebook says so and it is fun.
13. The importance of retainers.
So I am going to jump to:
14. The combat sequence
From page B24:
A. Each side rolls initiative (1d6).
B. The side that wins initiative acts first (if simultaneous all actions are performed by each side at the same time):
1. Morale checks, if needed.
2. Movement per round, meleed opponents may only move defensively (spell casters may not move and cast spells).
3. Missile fire combat:
a. choose targets
b. roll 1d20 to hit; adjust result by Dexterity adjustment, range, cover, and magic
c. DM rolls damage
4. Magic spells (roll saving throws as needed: 1d20).
5. Melee or hand-to-hand combat:
a. choose (or be attacked by) opponents
b. roll 1d20 to hit, adjust result by Strength adjustment and magic weapons
c. DM rolls damage; adjust result by Strength adjustment and magic weapons
C. The side with the next highest initiative acts second, and so on using the order given above, until all sides have completed melee.
D. The DM handles any surrenders, retreats, etc. as the occur.
Now a couple of interesting things about this:
1. It is very "wargamey" as one would expect given its pedigree.
2. It reflects a simple system that fits extremely well with the abstract nature of B/X combat. As JB over on his bog (B/X Blackrazor) succinctly says: "The initiative roll simply determines whose damage gets applied first...not necessarily who swings first."
3. With the movement phase coming at the beginning of the sequence it allows the combat participants to try to get the right resources into the right position.
4. It says in both the missile combat and melee combat phases, "DM rolls damage". This one is kinda neat but I would presume that 99.9% of DMs (including myself) let the players roll damage. Why would the rules say that the DM should roll the damage? I would assume that using today's vocabulary it is to keep metagame thinking out of the action. DMs should instead keep the numbers hidden and describe and roleplay the damage instead.
5. When engaged in melee it only allows for Defensive Movement (Fighting Withdrawals and Retreats). This has two effects: first it is a very simple way of keeping track of "attacks of opportunity" and it makes for an interesting decision process for managing resources (in this case hit points). When is it the right time to start withdrawing? When are the bonuses for your opponent to hit you outweighed by your need to escape?
Why do I like the B/X combat sequence? Because it is simple and fast but allows options for the aspects that are most important to B/X - the marshaling of resources.