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One common element in (most?) all RPGs is the attribute system. They may differ in what they include, but they all have them.
The traditional DnD model has:
Strength - physical strength
Dexterity - agility and speed
Constitution - ability to resist damage and disease
Intelligence - mental acuity
Wisdom - intuition and sense of things around himself
Charisma - force of personality and physical attractiveness
You typically see some variant on this in most RPGs. Most MMOs don't use Charisma, but the others are usually there. Wisdom is sometimes replaced with "Spirit" or some similar term.
Some condense these six even more, some split them. You'll sometimes see Speed split off from Dexterity. Sometimes you'll see Constitution split into things like Endurance, Toughness, etc. Many MMOs also have secondary attributes that are either derived from the core attributes or separate, things like crit chance, crit power, etc. Sometimes Health, Stamina, Mana are derived from core stats, sometimes they're calculated on their own.
What do you think is the best course to take for an attribute system? Is more better? It certainly gives you a better ability to control the small details without affecting too much, but at what point does it become too cumbersome and complicated? What do you think are the essential elements?
Health is usually one of the most important and universal of all attributes. Then you have some sort of energy mechanic to limit the use of powers/abilitites, which can either be done through Stamina, Mana, cooldowns, or some combination of the three.
You typically need an attribute to determine the strength of attacks, whether physical or magical (usually covered by Strength and Intelligence, respectively). The accuracy of these attacks can either be covered by a skill or an attribute, or a combination of the two. Some systems combine the damage and accuracy attributes, but I don't particilarly like that approach.
Defense/resistance is usually covered either by equipment (armor) or an attribute such as Endurance/Toughness/Constitution.
Top MMOs: Asheron's Call, Shadowbane, EVE Online, Planetside
Played: Pretty much everything at one point or another