Actually, wasn't the Wikipedia article but the KH Wiki which is fan-updated and I recognized that. Also, Nomura is inconsistent, I get that to. It was more of a refresher since it has been a while since I last touched the older KH games.

I think saying that strength is qualitative, not quantitative, exactly alludes to what you said. As is the strength of the heart cannot be measured like distance or height. Just like if one ask what kind of person I am, it would be a qualitative analysis not quantitative (most likely anyways). Also, I spoke in terms of correlation not causation. Moreover, it could be argued that all of such factors that you mentioned could be considered, or even productive of, a strong will. You can easily say that Sora has a strong will because of his friends and desire to help them, Xehanort will is strong because he wants to rule Kingdom Hearts, etc...I never said that their reasons for having a strong heart were the same, just that they all had strong wills. What makes a will strong is equally different from person to person. That just leads us back in circles and hasn't really solved much. However, I think that speaking of strong hearts in terms of will for us is more productive for us (Western audience) since it is easier to speak of people of different "moral agency" (e.g. the bad guys and the good guys) in terms of will than of heart. Saying that the antagonist also has a strong heart is a bit jarring, than saying he has a strong will.