I offer this style guide, with advice reverse-engineered from the D&D 4e core rulebooks, to improve the quality of third-party writing about and for D&D 4e/GSL products. Please use the comments feature to suggest additions or corrections.

Abilities. Capitalize the names of the six character abilities, both when spelling them out and when abbreviating them. Abbreviate ability names when placing them after skill names to indicate the skills’ key abilities, in the prerequisite lines of feat descriptions, and in the ability score section of monster and NPC stat blocks. Spell out ability names in power stat blocks.

You don’t take an armor check penalty to ability checks (such as a Strength check to break down a door or a Dexterity check to determine initiative in combat).

Attack: Charisma vs. AC
Hit: 2[W] + Charisma modifier damage.

“Armor Class.” Capitalize the term “Armor Class” wherever it appears. Abbreviate “Armor Class” as “AC,” with no periods.

Class features. Set class feature names in “title caps” wherever they appear.

The extra damage dice from your Hunter’s Quarry class feature increase from d6s to d8s.

Class names (including build names). When you refer to a class in a sentence, do not capitalize the class’s name (unless English grammar otherwise demands capitalization).

Elves and razorclaw shifters make excellent pursuing avengers, and devas excel as isolating avengers.

Class tag lines. A class tag line should usually consist of a quotation that might come from a member of that class. As quotations, these tag lines should appear within quotation marks. Most will consist of a complete sentence or two, and should be punctuated accordingly. Set class tag lines in bold and italic type, between the class name and the first paragraph describing that class.

“Have courage, my friends! Pelor favors us today!”

Defenses. Capitalize the names of defenses, but not the word “defense.” In descriptive prose, always use the word “defense” along with the defense name. Within stat blocks, omit the word “defense” from attack strings, but include it when describing power effects.

Your Intimidate checks are made against a target’s Will defense …

Attack: Charisma vs. Will
Effect: The target takes a –2 penalty to Will defense (save ends).

“Difficulty Class.” Capitalize the term “Difficulty Class” wherever it appears. Abbreviate “Difficulty Class” as “DC,” with no periods.

“Dungeon Master.” Capitalize the term “Dungeon Master” wherever it appears. Abbreviate “Dungeon Master” as “DM,” with no periods.

Feats. Set feat names in “title caps” wherever they appear.

… you gain Defensive Mobility as a bonus feat.

Keywords. When you refer to a keyword in a sentence, do not capitalize the keyword (unless English grammar otherwise demands capitalization).

You gain a +1 feat bonus to attack rolls and damage rolls when you use a power that has the fire or fear keyword.

Magic Items. Italicize the names of magic items, but do not capitalize them unless otherwise required by English grammar (for example, when proper noun appears in the magic item’s name). If a magic item grants a numerical modifier, treat that modifier as part of the item’s name, placing it at the beginning of the name.

For example, a cleric could pick up and use a +3 staff of fiery might as a melee weapon …

Monster names. When you refer to a monster in a sentence, do not capitalize the monster’s name unless it is a proper noun (or unless English grammar otherwise demands capitalization).

Baphomet’s minotaur cultists often summon goristros …

Paragon path tag lines. Set paragon path tag lines in italics. A paragon path tag line should consist of a quotation typical of a character pursuing that path.

“I have been touched by the power of the fey, and it drove me mad … but the power I gained from the madness has rendered me sane.”

Plane names. Capitalize plane names—excluding the word “the”—wherever they appear. Capitalize “the” only at the beginning of a line or sentence, or if otherwise demanded by English grammar.

They are numerous in the Abyss and the Elemental Chaos, but thankfully rare in the natural world.

Pluralizing dice expressions. Always form plurals by adding s or es, never by adding an apostrophe and s.

The extra damage dice from your Hunter’s Quarry class feature increase from d6s to d8s.

Power names. At the top of a stat block, set a power name in Roman type and use “title case.” Otherwise, set a power name in italics and use lower case (unless English grammar otherwise demands capitalization). Please note that some racial traits grant powers with identical names. In these cases, determine whether you wish to refer to the power (typical) or the racial trait (unusual), and typeset accordingly.

The Dragon Breath racial trait grants the dragon breath power.

Punctuating power descriptions. In a power stat block, conclude each hit, miss, effect, sustain, requirement, or special line with a period. Do not set a period at the end of any target or attack line.

Requirement: You must be wielding a crossbow, a light blade, or a sling.
Target: One creature
Attack: Dexterity vs. Reflex
Hit: 2[W] + Dexterity modifier damage.
Special: If this attack is made before the target has acted in the encounter, increase the weapon damage to 3[W].

Race names. When you refer to a race in a sentence, do not capitalize the race’s name (unless English grammar demands capitalization).

Love of stories inspires many gnome heroes to become bards.

Gnomes’ love of stories inspires many of them to become bards.

Racial tag lines. A racial tag line should consist of a sentence fragment; as a fragment, it should not end with a period. Set the racial tag line in bold italics between the race’s name and its stat block. Also, please note the proper terminology: “racial traits,” not “racial features.” Classes have features; races have traits.

Masters of stone and iron, dauntless and unyielding in the face of adversity

Racial traits. Set racial trait names in “title caps” wherever they appear. Please note that some racial traits grant powers with identical names. In these cases, determine whether you wish to refer to the power (typical) or the racial trait (unusual), and typeset accordingly.

The Dragon Breath racial trait grants the dragon breath power.

“Residuum.” Set the word “residuum” in italics. Do not capitalize it unless English grammar requires capitalization.

Residuum is the magical substance that results from using the Disenchant Magic Item ritual … In some exotic locales, residuum is traded as currency …

Rituals. Capitalize ritual names wherever they appear.

The Arcane Lock remains until you dismiss it, the door is destroyed, or until the ward is defeated, such as by the Knock ritual …

Sizes. Capitalize size category labels wherever they appear.

You conjure a Medium flaming sphere in an unoccupied square within range …

Skills. Capitalize skill names wherever they appear.

In another skill challenge, you might use Nature checks and Perception checks to track cultists through a jungle …