What are "the most important rules" in D&D? This is a question I have asked myself recently because I'm trying to boil the game down to something so simple that my four-year-old son can play with me. Part of that simplification means simply changing the game to a board-game format where he can see the dungeon in front of him (visualizing verbal descriptions really isn't his thing yet), but this process still has lead me to believe that "the most important rules" in D&D are often overlooked, and have been overlooked in recent decades and gone unwritten entirely in the 5E Basic Rules (as far as v0.2) and Starter Set.
(And yes, of course the Basic Rules are a work in progress. But even if every single issue I discuss below is addressed by the full Basic Rules v1.0 released in December or January, it seems very odd to me to release them in such a backwards fashion)
The most important rules are not stats, or race, or class, or alignment, or anything like that. In my first iteration of Ultra-Basic D&D (or Ultra-Basic I) characters only had two stats - Gold and Hit Points. The object of the game was to collect 10 gold pieces before losing your Hit Points. That's it. And that played pretty well. We will introduce more options later.
The most important rules do not include a long and detailed monster list either. Do you need monsters? Sure, but not fifty different kinds. Those are "nice to have" extras, but you can start playing D&D with a very short list.
Is combat important? Well yes, but it doesn't have to be complicated. In Ultra-Basic I combat is resolved with a d6 and everyone has the same flat 1-3 miss, 4-6 hit probability. In my next version (Ultra-Basic II) I will introduce classes with variable to-hit chances on a d20 (Fighters will hit on 8+, Wizards on 13+, etc.), but still ignore enemy AC or all modifiers. Maaaaaybe we will introduce Advantage, but probably not until Ultra-Basic III is released when he's 6 or 7 years old. Enemies are always goblins that die in one hit, and in Ultra-Basic II we will have more than one monster type, some of whom are tougher.
But, we are getting ahead of ourselves! Combat is one of the last things we resolve in D&D. Let's see the steps I had to go through to get there.
Step 1: Draw a Dungeon
Graph paper, pencils, some ink and crayon to fill in details. Just a basic dungeon. But isn't it amazing that the 5E Start Set and Basic Rules have no guidance on how to do this? How many rooms are good? Should I avoid linear maps? How often should traps occur? There is nothing on this. The dungeons in Phandelvar are actually pretty decent examples of the form, but how can a game called Dungeons & Dragons have no rules whatsoever on making dungeons?
Hopefully this will be rectified once the DMG is released and the Basic Rules are updated to include basic DM guidance, but I what are kids who just have the Starter Set supposed to do once Phandelvar is fully explored? I'm sure they'll muddle through, as I did as a kid, but still, something would be better than nothing.
Step 2: Explore the Dungeon
This step includes a lot of sub-steps, but the Basic Rules only touch on some of them. The Basic Rules name exploration as one of the key pillars of the game (good!), and then provide rules for movement (including stealth), finding traps, and vision. Good so far.
But what about encounters? Where are the monsters? What are they doing? Are they friendly or not?
Whom do you meet?
To make Ultra-Basic I I had to come up with a Random Encounter/Dungeon Room Table, and basically cribbed the one from AD&D. A simple d10 roll for each room had odds for Monsters, Trap, Special, Secrets, Treasure and Empty. I then needed sub-rules for what sort of "Special" results you can get (Dungeon Alphabet to the rescue!), how many monsters there were, and treasure. (This is related to drawing a dungeon - filling it in. But in Ultra-Basic the dungeon content is generated as you play, not pre-generated by a DM. Four-year-olds don't appreciate well crafted dungeons that "make sense" and do appreciate very much spinning the "What's in the room?" spinner.)
Are they helpful?
In my mind, what really makes D&D a "roleplaying game in a fantasy world" is that not every encounter has to be a fight. Many monsters can be spoken to or reasoned with. Many "monsters" in the old encounter tables aren't even really monsters. Are halflings monsters? In Ultra-Basic I there's only one type of Monster (Goblins) and they're always unfriendly but in Ultra-Basic II that will change, and this is something even a five-year-old who cannot read or understand "leveling up" can use and play with. And it's lacking entirely from the 5E rules, as far as I can tell. This is amazing to me. Is the DM just supposed to use fiat to decide each encounter? That's the road leads towards railroads, in my opinion, whereas Reaction Tables leads towards a complex world that surprises even the DM.
What do we get?
Finally, we needed rules for figuring out what sort of rewards you get. Ultra-Basic I only has two rewards - Gold and Healing Potions. That will probably stay the same in Ultra-Basic II. Maybe we add silver and gems (for math and variety!) in Ultra-Basic III in a couple years.
Step 3: Define the End Game
What's the point of D&D? When's the game over? How do you know if you're making progress?
This is an interesting this - the 5E rules have no purpose. Just a hamster loop of XP and levels up until you're 20th level. Then ... what? What was it all for?
Games don't need a point. No one really asks why you're trying to checkmate the opponent's king in chess. Maybe for some people reaching 20th level is good enough. But that seems a bit empty to me, considering that you're expected to invest mental energy into the persona of a character. What does that character want out of life?
In the original version of D&D the implied end-game was to become a Lord and build a Keep, and then shephard the next generation of adventurers. Gary himself mentioned there was no reward for reaching 20th level other than recognition from your peers for accomplishing that difficult thing. I think the real reward people were looking for was to see their PC graduate into the constellation of "important NPCs" that moved and shook the game world. "King Azoun and Vandergahast have put out a call". And so on.
In Ultra-Basic I the goal is to collect 10 gold pieces. That's it. Like a board game there's no continuity between one game and the next, and that's fine for now. I think continuity will wait for Ultra-Basic III (there will be no Ultra-Basic IV), where perhaps the goal will be to collect three Dragon Gems (but you can't even try to collect them until you're Level 5).
But anyway, End Games are flexible. Maybe you want a campaign where the end game is to kill a Dragon King, or become one, or decisively defeat a Dragon Army, or just build a Keep and tutor the next generation of adventurers. But you should have one, and 5E doesn't even discuss the need. That seems weird to me.
Conclusion: Here are my "Most Important Rules in D&D"
1. DM Procedures for making dungeons (and later, Wilderness Maps, Kingdoms, Planes, etc.)
2. Procedure for exploration and random encounters
3. Reaction Tables - what sort of encounter is it?
4. Combat (and maybe a few spells) (KISS).
5. A progress metric (Can be Gold Pieces, XP, or User Defined. But have one).
Together these rules are the procedure for Playing D&D. Set-up, Exploration, Conflict Resolution, and Winning Conditions. All the options for character building and monsters to fight comes later, in my opinion, so it seems weird to me that D&D 5th Edition (as released so far) contains so much of the latter and with such obvious gaps in the former.