I'm late to the party but I just want to give my two cents even though some of them echo previous users' thoughts. Just focusing on the primary stats.
Strength
How physically strong your character is, the amount of force of an attack i.e. how much damage it does IF it hits, not your ability to hit something as commonly seen in RPGs. Strong characters should also be able to wear heavier armour with less penalties (on action points for example).
Outside of combat this is used for anything physical, eg. lifting things, breaking doors, etc.
Dexterity
How skillful a character is, this should be for determining a characters ability to hit for both melee and ranged weapons, however if I learn how to use a sword that does not mean I am automatically skillful with a spear or even a bow, so I would like to see this weapon specific but that's probably not feasible. It should also affect things like disarming traps, opening locks, etc. as you need to be skilled in such matters to solve them, again this is a specific type of dexterity though so would need to be combined with secondary skills.
Intelligence
This should not be limited to magic users/number of spells etc. (I want to say it affects a character's ability to perceive things but I guess Perception does that.) Intelligence should also affect things like the number of skills one can learn and how quickly they can learn new skills. Like dexterity, it should be necessary for skills such as opening locks and disarming traps.
Speed
How agile a character is, this could determine the number of action points available and also the consumption of action points. It should also determine the order of movement for turn-based parts. A character with high speed should also be harder to hit.
Perception
This is an unusual one, a characters ability to sense things outside of the ordinary. Magic users possibly need to have a high in order to be able to know about and use magic. It also sounds like it would be useful for detecting hidden creatures. I like the sound of it.
Constitution
How tough a character is. I don't particularly like this as a core statistic or the idea of hit-points but it does make things easier. Still, it you're hit by a sword I don't care how tough you are, armour is far more important.
