The system should preserve the relative rank order of specific abilities among characters, presuming equal initial talent and equal attention to advancement. I.e. if one character starts out more stealthy than another, or a better shot, it shouldn't be possible for the less skilled character to overtake the more skilled one by accumulating equal experience, unless the more skilled one neglects to advance that skill, or was deliberately bought as less naturally talented at it. Insensitive to timing The system shouldn't distinguish between character that have advanced through experience and characters that are simply created as being more experienced. Equivalent to training Although for some fields, experience attainable through the school of hard knocks ought to translate to experience from adventuring, for many abilities non-adventuring time spent training or on the job ought to be treated equivalently, and the system should provide for it. E.g. it should be perfectly possible to design a bright NPC high-school student, calculate how much experience she would get from attending college, entering graduate school, completing her PhD, and spending twenty years as a professor, apply it to the character, and arrive at an expert in the field.
Concrete gives results that can be interpreted in quantitative game-world terms.
Robust gives reasonable results at all power levels and combinations of power levels handles unlikely cases as well as likely ones. Easy to extrapolate Intuitive It should be easy to intuit the probabilities of any simple course of action, given familiarity with the game system. To such an extent as is possible.