Draco, you are talking about "natural talents" for example. Anything like this could only work, if the various attributes couldn't be maxed by each and every dwarf. Correct?
While not explicitly stated, that would most likely happen (given the Skill Rust). If you "max out" one stat and start working on another, you'd acquire rust on it. Eventually you'd not be able to maintain all of the max/near max skills that you have, but the idea is that one dwarf's maximum toughness (for example) isn't the same as another dwarf. Both might be "tougher than average" (have a higher max than Urist and be at it) but one guy will give into pain before the other.
Edit, skills too. For example, maybe McCarpenter has a lower potential for carpentry than average, and while good, can never be "the best" (in this respect, I'd place Legendary +5 as the highest possible potential, personally I think the lower bound would be around "professional" with your average at Grand Master; reasoning: legendaries are
f-ing legendary, these are the guys who's potential is higher than the norm, your generic dude shouldn't rise above Grand Master because he just isn't Legendary, while someone who works at it, but has a low potential can still be a "professional" at the job--they'll produce fine to superior furniture on average, where as your generic average dwarf makes Exceptional pieces*).
*This sounds high to me, i.e. the average should be professional, everyone is a "white collar worker" with your above norms getting up to Grand Master, and only the super-dwarves (artifact makers) achieve legendary status, which modifies their potential to a new bell curve from Legendary +0 to Legendary +5 with the average around +2.5: a McDwarfy who had a potential of Grand Master could then achieve up to Legendary +5, while your Professionals would get +2 to +3. Unfortunately the lower bound on this would end up being around "competent." Actually, no, I think that works: your dwarves who "just suck" at some skill can still be
competent at it with experience, and those who have normal/average potential can be
professionals at their peak, while the select few achieve
master or
grand master. This makes linguistic sense, cuts down on the "champion spam" and "everyone's legendary!" problems, as well as reduces the need for recruiting peasants to make your furniture: you find a farm boy and apprentice him, but he proves to be lackluster, but he can knock out cheap furniture for the masses.