Class Yays and Nays

This covers design errors that are often made. Some of these are conventions (e.g. "round-counting" doesn't break the game but is at odds with 5e design philosophy), whilst others are just wrong.

Terminology

 * Class Features are "Features", not "Abilities" or "Skills".
 * Strength, Constitution etc are "Abilities", not "Stats". "Statistics" refers to all the game numbers of a game entity.
 * "Hit Points" - not "Health" or "Life".
 * Class names are (usually) not proper nouns.
 * Feature names and Sub-Class names are (usually) proper nouns.
 * Spell names are in lower-case italics.

Recharge

 * Wrong: "before needing to take a short/long rest", "between short/long rests", "per short/long rest" - the exact recharge point is ambiguous.
 * Correct: "You need to finish a short/long rest before you can use this feature again" - clearly states when you get the feature back.

Targeting

 * Features and spells can target "creatures", "creatures you choose", "friendly creatures".
 * While creatures can have a "hostile" attitude towards you, you would normally use the phrase "creatures you choose" to avoid ambiguity.

Duration

 * Do use the standardized durations: 1 round, 1 minute (the length of one combat), 10 minutes (one "exploration phase"), 1 hour, etc.
 * Do allow a creature to repeat a saving throw for applying effects on enemies for an indeterminate length.
 * Don't use round-counting, e.g. "lasts for 3 turns", or "1d4 turns". If you must have a variable length, consider using a d6 recharge (the method used in the Monster Manual)
 * Don't have features that last for the duration of the "combat" or "encounter", as this is not defined (but see "1 minute" duration above); similarly do not have "at the start of combat/encounter" (but you could have something trigger on an initiative roll).

Action

 * Don't forget to say how the feature is used. Is it an action, a bonus action, a reaction? Is it triggered on another action, or does it work in a general (non-combat) way?

Damage-For-Benefit

 * Be careful with Features that grant you a benefit if you take damage, as it's trivial to arrange to be damaged by a willing player or Non-Player Character.
 * Do have the benefit only work against the creature that caused the damage.

Regaining Hit Points

 * Be careful not to disrupt the hit point economy of using hit dice to regain hit points during a short rest. These examples allow a character to regain all their hit points too easily:
 * Don't have unconditional regeneration.
 * Don't have anything "count as" a long rest.
 * Don't have at-will "cure" effects.
 * DO use temporary hit points.

XP and Treasure

 * Don't make features that change the way XP, gold, or treasure is granted. These rewards are entirely the purview of the Dungeon Master.

Feats

 * From WoTC: "...never assume that a particular Feat will be a part of the game. For instance, a class can't refer to a Feat and Feats should never be granted as Class Features."

In General

 * Do give your Class Features to help with combat, exploration, and interaction
 * Do think of new Class Features rather than copying Features considered unique to a particular class (e.g. the Barbarian's Rage)
 * Don't make your class concept too narrow. Particularly when adapting a character from another work of fiction, don't tailor the class to fit that character exactly: pull back and consider a broader scope with options that could make that character.
 * Don't have negative features that punish players for picking a certain class.

Other

 * Don't invest a lot of writing in describing the effects of edge cases that might never crop up. For example, in one class, I saw a big paragraph about what would happen if they cast a spell without using their focus. Such a situation is a rarity. It would be better to keep the effect of that situation simple, so the class isn't cluttered with nonessential information.