Table Rules
DMG page 235, "There are the rules of the game, and there are table rules for how the game is played. For instance, they (players) need to know...how to treat a cocked die." Topics listed include Table Talk, Dice Rolling, Rules Discussion, Metagame Thinking, etc. Chapter 8 of the DMG touches on unwritten or soft rules of the game. They discuss things tantamount to style, expectations, and the etiquette. The books make suggestions, but these are things the GM takes responsibility for.
Declaring Actions
Once you've declared your action, you can't change it. No do-overs, no redos, no retracting what you've said. No retcons. If you recognize a mistake two rounds later or after something has been resolved, we're not going back and correcting it unless it's life or death. Otherwise, we can talk about it outside the game and try to do better in the future.
Joke Characters
No joke characters. Joke characters spoil the tone (mood) of the game. Try not to spoil the tone.
PvP
No PvP (player vs. player). Adversarial play makes your characters unlikable to me, the GM. If your characters are unlikeable, it makes it harder to care about preparing and running the game.
Evil PCs
No evil PCs. Evil PCs are something I don't like about this hobby. It's not fun for me. If you're going to make a character who's a jerk, give them some redeemable quality to make them likeable. If you do something heinous enough, I reserve the right to take your PC and make them mine, and make them an NPC villain in the setting.
PC Names
No joke names, no dumb fantasy names. I need to be able to spell, proncounce, and remember your character name. I am the GM and I portray all NPCs. Your parents are NPCs. Therefore, I have veto powers for all PC names, and it goes like this "my NPC would not have named their kid that. Pick another name or I open up the Bible."
Character Builds
Some players have fun by finding exploits with powers and abilities available in the game and mixing and matching them to create overpowered combinations to trivialize challenges. These are called character builds. Sometimes these builds are called broken because they spoil the intended way the game should play or function. This makes more work for me, the DM, and I don't like it. If you bring a build to my game, I reserve the right to hand you a pre-generated character (a pregen) to play instead, I reserve the right to nerf your character without justification, I reserve the right to do something arbitrary and malicious up to and including stating "rocks fall, your character is dead.", and I reserve the right to remove you from my game or group.
Electronics
I prefer pen and paper. If you're relying on D&D Beyond or a similar application to track your character or understand the rules for how your character works, here's a warning: if that app doesn't work or is slow, I'm not waiting on it.
Table Talk
First rule about speaking at the table: Don't be a dick. If you get mad, it's OK to excuse yourself. It's OK to ask for the GM to call for a break. It's OK to leave a session and come back next week. Second, if it's unclear if you're speaking in-character or out-of-character, I am going to treat what you've said as in-character. No jokes or side-conversations. No phones. No building dice towers. No drawing. These things spoil immersion and slow down the game. Pay attention and be ready.
Player Narration
Players narrate their own stuff. You tell us what your actions look like. Tell us what we see, hear, etc. Use natural language rather than game terminology. For example, don't say "I use my bonus action to rage, I use 20 ft of movement to move to the goblin, and I use my action to attack." Instead, you can say "you see Conan go into a rage, run, and swing his sword at the goblin's head." Don't say "I cast fireball." Say "you see Samson conjure a small flame and hurl it into the center of the goblin hoard and it blossoms into a great big hemisphere of flame." Here's another rule: Description or roleplay first, roll second.
No Rules Discussion During a Game
During the game, there is no asking rules questions for clarity, no reading rules allowed or describing them, no asking the GM for their reasoning for a ruling. The exceptions are if the situation is life or death. Save it for outside the game. Do your best to use the rules correctly and we'll use the honor system. If the rules are unclear or lacking, or if our recollection of the rules is incomplete, I'm going to make a ruling and move on with the game.
No Breaking Character (the Metagaming Rule)
5e DMG page 235 describes Metagame Thinking as "thinking about the game as a game." Essentially treating the game like a game rather than a roleplay exercise. Metagaming is essentially what happens when you break character or roleplay poorly.
For instance, your character doesn't have a concept of a balanced encounter. If you are playing your character cock-sure that the encounters, traps, hazards, etc., will be suited to your character's powers and abilities, you're not playing your character like they care what happens to them. You're playing your character like an idiot, a madman, or like someone with no sense of self-preservation.
Here's another example of metagaming. A combat round represents 6 seconds of real time. It is therefore metagaming to have a discussion about what to do and how to do it mid-encounter if that discussion is out-of-character. If it's in-character, you must wait for your turn to speak, and you can only speak an amount that is reasonable for a 6-second time span.
The best tip to avoid metagaming is to stay in character as much as possible. Every time you break character, you're breaking immersion and you're slowing the game down. I challenge you to try to go without breaking character as much as possible like Liam O'Brien from Critical Role.
No Speech or Insight Checks, Limited to No Ability or Skill Checks
Per the DMG, page 236, under the subheading Ignoring the Dice, the 5e DMG says that it is valid for a Dungeon Master to "...use dice as rarely as possible. Some DM's use them only during combat and determine success or failure as they like in other situations."
I believe that the player should use their own intelligence, wisdom, charisma, and luck. This is called player skill over character skill. Treat the NPCs like real people, not like an opportunity to roll-to-solve (tm) a problem; Try to find a solution in-character by using curiosity and creativity. Try to make the NPCs happy or satisfy their needs to get something in exchange. I see game mechanics for social interaction as unnecessary, and the results of their use often feel contrived and unsatisfying. I also think they disincentive player engagement.
Also refer to these passages for social interactions specifically:
-PHB page 186 Results of Roleplaying: "The DM uses your character's action and attitudes to determine how an NPC reacts..." "Pay close attention to the DM's portrayal of the NPC's mood, dialogue, and personality. You might be able to determine an NPC's personality traits, ideals, flaws, and bonds, then play on them to influence the NPC's attitude." "Interactions in D&D are much like interactions in real life..."
-DMG page 244 Social Interaction: "Some DMs prefer to run social interaction as a free-form roleplaying exercise, where dice rarely come into play..."
Interpreting the Dice
5e DMG page 242 has allowances for failing forward or succeeding at a cost if the roll just shy of a difficulty class, for degrees of success or failure where additional effects can occur for when a roll is +/- 5 of a difficulty class, and treating 1s and 20s as special even outside of combat. I will use these situationally and they will be entirely case-by-case. I won't use these as opportunities to be generous or cruel to the players; instead, I'll use them to make the game more interesting or to manage the pacing of the game.
Rules Lite
Conversation is the medium of the game, not the rules or mechanics. I will repeat this for emphasis. Conversation is the medium of the game, not the rules or the mechanics. I as the GM am the referee of the rules, and I will be using them sparingly because I don't like most of them. If something can reasonably succeed, I'll probably just say it works. If something is stupidly impossible, I'll probably just say it fails. Consider the passages below:
-DMG page 4: "And as a referee, the DM interprets the rules and decides when to abide by them and when to change them."
-DMG page 4: "...but the rules aren't in charge. You're the DM, and you are in charge of the game."
-DMG page 4, How to Use This Book: "The last part helps you to adjudicate the rules of the game and modify them to suit the style of your campaign."
-DMG page 5 subheading Part 3: Master of Rules continued: "As a referee, the DM acts as a mediator between the rules and the players. A player tells the DM what he or she wants to do, and the DM determines whether it is successful or not, in some cases asking the player to make a die roll to determine success..." and "the rules don't account for every possible situation...How you determine the outcome of this action is up to you."
Grounded Fiction
Fiction / fantasy / sci-fi doesn't mean anything goes. The logic of the make-believe world is based on out real world (but is not strictly realistic or simulative). Assume if something can work in real life, it works like that in the game. The reason why we make this assumption is because the real world is our shared point of reference. In other words, don't ask what the game rules allow, ask what the world allows. This makes the game intuitive. Apply our basic understanding of the real world to the make-believe world to understand places, people and character actions. This is how I will make rulings and this is how I expect you to decide what your character can do in a given situation.
More to Come
This is a start. I expect to find more problems with the game and to find more solutions to them as I go were I to actually run 5e. I expect that this post could be revisited and expanded upon by a lot.