Role-playing games are one of the largest and potentially most complicated genres. Depending on who you ask, the most important thing will be the story, the look and feel, or the combat, and these are all important. But to me, they are all influenced by the game system designs, so that should come first.

So go to game design 101: who is your target audience. For "ER", an RPG/Simulation based on the television show, the target audience was viewer of the show, who were mostly non-gamers. My RPG system had to be simple, accessible, understandable to these soccer moms. Instead of using a scale like D&D's or GURPS 3d6 based 3-18, I used 100 as average; higher is above average, lower below. Why? Every school child takes IQ tests; the players knew this scale. Remember, the numbers don't matter. If 10.5 is average (the average result of 3d6) then multiply all variables by 100/10.5 to true the scales to 100.

Then decide what stats to include. For a complicated RPG, you might have attributes, races, classes, spells, skills, inventory items, environments, and synergies and interactions. How many of these do you use? That's up to you. But consider the target audience.

I recommend using additive rather than multiplicative synergies. When you add six numbers together, you are pretty sure what the order of magnitude of the result will be. When you multiply six numbers together, the result can be enormous. If the value is (say) damage caused, you don't want a solo player character to one shot a boss, so you'll have to give the boss a multiple of the maximum damage. That means that the less well built character, who might only be multiplying four numbers together, will never beat that boss. That leads to unhappy players, who quit and badmouth the game and will never buy the sequel.

Finally, the first thing I have to repeat to new game designers is: your role is the same as the bull in the bull fight. You are expected to present a challenge, once in a while gore a toreador, but if the players don't win, they have less fun. You can always kill a player character: Damage(Max_Health) is an easy macro. So don't do it unless the players deserve it. You can always scale an opponent's hit points to the amount of the expected or maximum damage possible. You can scale the monster's damage to the player's maximum hit points. You can use difficulty levels to adjust those numbers.

A good RPG system won't guarantee a good RPG. But it's a lot harder to tell your story if a bad system makes it too easy or hard for the players to engage.