From what I understand of the Incubator's system, i take potential to mean 'how much have you suffered in your life'
This is probably why Kyuubey found Madoka's potential to be so unordinary - Other than her drunk mother (who isn't even that bad, might i add), her life experience does not fit the massive amount of potential she has.
Hitomi's potential is probably so low because she hasn't suffered in life: Her family is loaded, she has a good social circle, and she has a good school life. This experience would naturally give less potential than, say, having your mother die of a terminal illness.
My read is that potential is just that: How much capacity you have as a person to change the future, for good or ill, and specifically how much capacity you have to change the lives of everyone around you. Teenagers will naturally have more then, both because they have longer left to live, and because they're still changing as people; There are a number of ways their lives could go, even if a lot of "who" they are has settled in.
Beyond that, the sort of people who, by talent and effort or luck of birth and circumstance, attract the attention of others will naturally have more potential. Someone who has no drive to change the world around them, or who seeks to maintain things as they are, will have a sharply limited potential whether or not they are in a position of power. I would imagine personal suffering almost certainly affects that, as it can be a powerful source of drive to change your own and others lives, but only those who can make their suffering a source of strength, rather than being destroyed by their experiences, will grow in potential from that.
For reference, I read grief a little differently than Sabrina does IC. I don't see it as "personal emotion and suffering" so much as
curses, specifically the ability to inflict suffering, on oneself yes, but also including the potential to inflict suffering on others.
I'd read Madoka here having high potential, then, for a few reasons: As far, as metaphysics, its because she's connected to her lives in other realities, and whatever the soul really is is flowing between that and affecting far more people than it "should." But its also higher because she she has both her own vague recollections and Homura's outright help keeping her away from wishes she'd regret or which would be limited in impact, so she's more thoughtful and less impulsive with her wish than she otherwise would be.
Aside, I'd also headcanon that people who are generally
weird in some way or another would probably have more potential on average because they come to the world from perspectives outside the normal, and so they naturally have abilities to affect the lives of others in ways that otherwise wouldn't happen. That said, that
is a headcanon based in personal logic, rather than supported by the text anywhere I'm aware of.
Finally, it's occurred to me that magical girls dying early also limits their ability to enact real change. Which makes the system and the way it uses up girls a force of
stasis. Given that Kyuubey is often read as a metaphor for society, I would think that is probably not an accident.