It's a difficult subject. If you make the attributes granular, you can simulate anything, but end up with some stats with niche and/or low utilization. Your system also becomes bloated and less comprehensive. If you a fewer but more broadly defined attributes, there can be some undesirable and non-intuitive results that result in a lot of bootstrapping. The attributes also mean little outside the context of a skill/class system. What I have here is intended to be used in a steampunk, Arcanum-like setting.
Attributes
- Finesse (Action point maximum, movement, reflexes, weapon attack, agility, initiative)
- Constitution (Action point regeneration, HP, health, bonus melee/thrown damage, encumbrance, endurance, strength)
- Reasoning (Skill quantity maximum, learn skill %, spell casting, knowledge checks
- Perception (Maximum range, detection, initiative, insight/assessment, overcome concealment)
- Resolve (Diplomacy, barter, will, effort resource, performance, concentration)
Skills
- Martial (Weapons, warfare, tactics, etc.)
- Arcane (Magic/Technology)
- Athletics (Dodging, climbing, swimming, acrobatics, etc.)
- Survival (Navigation, first aid, wilderness skills)
- Social (Diplomacy, trade, leadership, culture, language, etc.)
Skills represent reliability and training (number of dice rolled, take highest), attributes reflect natural ability (scope of dice). So, lets say a solider is examining an advanced firearm. They have d6 Reasoning and 2 Martial. They would roll 2d6 and take the highest to determine what they know about it. Theoretically, most attributes can be combined with most skills to simulate whatever you want. The separation of scope vs reliability also permits interesting match-ups, like a novice savant (1d10) vs an experienced every-man (2d6). Also, everyone, no matter how seasoned can still trip on their shoe strings--even if rarely. This division also gives a lot of options for modifying rolls through items, abilities, and effects.
I have only really two complaints with this. While combining Strength into Constitution is a huge improvement, it creates a situation where something small and tough (like a gnome) will also be strong without applying some kind of size modifier. This is less elegant than I like. The utilization spread is otherwise pretty good, with Finesse probably being the most weighted, and Resolve being the least. I created an extra expendable resource called "Effort", which is derived from Resolve. It allows you to add 1 Resolve die to certain rolls like, action point regeneration, or a single attack. Some uses might be contingent on certain talent/feat selections (like resisting a spell, or stabilizing on 0 HP). I have another mechanic for Perception, which allows a character to study a target and gain a relevant insight bonus.
I have several other iterations, and I constantly move between them, changing things. I've gone as low as 3 stats, or as high as 8. It's something that's very easy to be OCD about. It's one of those situations where perfect is the enemy of good.