(Edits to these "rules" are coming soon-ish!)
Generic Rules for Icosahedral Narrative Designs (the GRIND) is the system I plan to use for most of the RPG rules content posted on Monstrous Matters. It's inspired by the simplicity of character stats in the minimalist RPG Searchers of the Unknown. The hope is that minimal mental conversion will be necessary to use the rules in a variety of games that use Hit Dice and a d20 for combat, especially older ones.
Level is an approximation of a character's overall strength, including any special abilities. It works similarly to Hit Dice Equivalent in other games and may be used to determine the amount of Experience Points awarded for defeating a character. If a character is of a special class, such as a Magic-user, this will be indicated after their level so that appropriate abilities (like spells known and prepared) can be assigned.
HD/Hit Dice/Hero Dice also deal with a character's strength, but special abilities aren't taken into account. HD are mostly about combat and may influence Attack Bonus and Saving Throws. Most importantly, though, they're used for determining Hit Points. HD are given in a generic "D" notation and can be altered according to the game your playing. White Box-style games will probably use a d6, Basic-type games a d8, and 3rd Edition-inspired stuff maybe as much as a d10. You can probably even increase this for use in 5th Edition games.
Defense is a character's ability to not be hit by an attack. It uses an ascending system with an unarmored character of average stats and skills having a Defense value of 10.
Attacks are noted by their bonuses and possibly some details about the weapon type. The Attacks listed are the ones a character can perform in a single round of action. Damage from attacks, like HD, is in a generic "D" notation and can be adjusted according to your game.
Skill, Expertise, and Mastery indicate increasing levels of ability or knowledge in a specific area. In most cases, only Mastery will be noted, as it has the most bearing on game results. In general, these three levels can be treated as advances of 1 each for d6-based skill systems, 4 each for d20-based skill systems, and 20 each for d100/percentile-based skill systems.
I'm always up for thoughts and recommendations on tweaks to this notation!