Classic 5th Edition

The world is not leveled to the player characters. Death is possible and so are great rewards. Until one "learns the ropes", caution and caginess are wise attitudes. Magical items are uncommon and not normally purchasable. The ability to cast spells. Most priests are humble servants or charlatans. Only a select few are granted the miracle of spell casting by their gods. Wizards are reclusive, feared, disturbers of the peace. Elves and Dwarves are alien, and indifferent to the follies of man.

Classic meaning in style of 70's and 80's D&D. Gritty, dark, "swords and sorcery", when elves were fighter/wizards and dwarves didn't dabble in the dark arts. A few changes and restrictions to 5th Edition Basic Rules to instill a particular flavor and style on the game I run. Not claiming any sort of superiorness. Really just showing how easy it is to and encouraging others to make 5ed their own.

Basic D&D 5th Edition PDF

Most of the flavor is in the races and classes (future blog posts). Below are some general house-rules.

Encounters

Every encounter starts with a reaction (Charisma Check) from one of characters in front rank. This check will guide NPC actions (flee, wait, parlay, attack) if they have surprise or initiative. Or, reactions to PCs friendly or neutral actions.

Group Initiative

Based on situation, their abilities, etc. Referee will split Character opponents into one or more groups and determine if each group acts before or after the PCs. E.g. slow zombies always act last, flyers and mounted act first, etc.

All Player Characters act together in any order they decide. Rarely the Referee may designate some PCs as being particularly slow or quick and place them in a separate group that acts independently.

[I have found few things that are as confusing, time wasting, in conflict with player desires, requiring all sorts of patches (delay, ready), and adding so little fun as d20 every round initiative. Having players act together also promotes teamwork and planning. FFG StarWars has interesting "middleground".]

Resting

Long Rest: Must be uninterrupted and truly restful. Including fire, warm food, relative comfort and safety. "Roughing it" unprepared or holing up in a random room of the Dread Dungeon of Doom ain't cutting it.

[The intent here is to not bring back "5 min workday" but rather provide Referee with option to have a stretch of dangerous wilderness in which characters must carefully husband resources. And return meaningfulness to the choice "press on for one more room or go back to town?" And / or treat dungeon complexes as Mythic places that must be escaped rather than napped in.]

Damage and Death

Consequences of  death / near-death: Brought back from death suffer permanent loss of one point of randomly chosen Ability. Two failed death saves, suffer permanent loss of one point of randomly chosen Ability. [Death doesn't have to be permanent, but it should be feared and avoided.]

Attempting to Knock Out: [Probably overcompensating for how annoyed I am with Pathfinder "subdual" damage"]

  - Attack with disadvantage unless using a Bludgeoning weapon.

  - Intelligence check required on final blow to avoid "accident".

  - Some creatures may resist (Con save). Some creatures will be immune to knock out damage.

Falling is deadly. Damage is cumulative: 1d6, 3d6, 6d6, 10d6, 15d6, > 50' auto-death.

Suffocating is too deadly. Not sure how to make it less so.

Spells

Magic is dangerous. Almost any mechanical method to avoid that will be ignored. e.g. Evoker school's ability to shape spells to avoid hurting friends. No. You must actually plan, communicate, and execute in game tactics.

No such spell as Identify.

Spare the Dying: provides 1 successful death save, not auto-stabilize

Probably no standard spell to revive the dead. 

Fireball is volumetric, similar with Lighting Bolt.

No Backgrounds

Instead make your own background; Pick one skill or tool proficiency and one language proficiency. Optionally select a profession.

No Feats

Skills

Referee will never call for a Social Skill check, role-play instead. Charisma checks will come into play often. Primarily in form of encounter reaction checks. [Social skills destroy games. Esp with explicitly called out Description based role-play they add nothing but boring, binary dice rolls. While robbing game of it's most unique feature, interaction.]

  - No Deception(Cha)

  - No Insight(Wis)

  - No Intimidation(Cha)

  - No Persuasion(Cha)

Literacy Skill
Literacy is not common. Characters must pass an Intelligence check to read or write a given piece of text. This skill allows you to add proficiency bonus to this check. Dwarves, Elves, and Wizards automatically pass this check for Common and their native language. There are several setting specific languages.