On Monsters and Races:
Remember that the GM is always right! Mastery is built to be flexible, not all-encompassing. If you, as a GM, feel that dwarves deserve stat mods – do it! It is because of this that the system doesn’t handle monsters or races. These things differ so much from one world to the next, that it’s on the shoulders of GMs and supplement writers to come up with rules for such things.

Skills and Abilities:
As a GM, the biggest task will be learning to classify skills and abilities and to call “not applicable” skills when players try to use them. This can be hard, as a player can fail simply because you don’t think farming ability helps classify wild plants. Combat can be especially bad, as powergamers will instantly create “Skill with Erchax the Longsword” and try to pass it off as a skill. Skills are more powerful than abilities, and should be much more specific, and very rarely cover simple attacking. Parrying, for example, makes a good skill. Counterattacking with a specific weapon is a good skill. Both of these are only usable in certain situations, which is why they’re skills and not abilities. Abilities represent professions, general skillsets and skill with weapons. When considering whether something is a skill or ability, also take into consideration the world and type of game you’re playing. Engineering might be an ability in a steampunk setting, but in a dark ages world it would probably only be a skill.

Don’t be afraid to say that certain actions are only possible by characters with points in a specific skill. I can’t do magic, so why should a farmer or a barbarian? This even applies to abilities, though less so. I may not be able to do a cartwheel unless I have acrobatics, for instance.

Lastly, say no to skills and abilities that don’t match up with a test! Don’t let your players re-pick different skills or abilities once you turn the first ones down! (unless you really think they deserve it) If players know that a bad choice will result in no extra dice, they’ll be more likely to pick the most realistic skills and abilities. Disallowing skills and abilities also forces players to get creative and tackle problems from different angles.

Weapons and Armor:
Weapons in mastery are a bit vague, much like the setting itself. The power level of weapons and armor depends on the technology of the setting and also how gritty the GM wants to run things. High weapon damage means more deadly combat, while high armor values tend to make for a more “epic” game (as high armor tends to mean a generally higher expected damage).

Remember that weapons are more than just that single number. Some weapons (such as daggers) are easier to wield in tight quarters and conceal. Others (such as flails) might be difficult to wield. Some will require special conditions to be effective (lances) or require more time to use (ranged weapons). If you feel that an attack deserves a damage modifier (such as a killing blow or a point-blank shot), propose it to the GM. And as always, modify these numbers as you see fit to make the game that is fun for you.

Proficiency is also important with both weapons and armor. In cases where a character attempts to use a weapon without any skill or ability, the GM may decide to impose any number of circumstance penalties. Armor, unlike weaponry, has no associated skill or ability (under the defaults), but the GM has full right to impose penalties on any number of things, especially if the wearer of the armor is unfamiliar with it.