I haven't seen any other threads, so I'm not aware of the background. However, I can venture a response to some of the points and questions in the thread.
Perhaps it's a question of how much the "political" must become "personal" for an individual to identify with the issue in question.
Some feminists (and activists against other forms of injustice) are motivated by their personal experience of discrimination. Others may feel they have never really been affected by the issue themselves, but they can empathise with others who have and are sufficiently concerned to speak up about it.
Yet others may be somewhere in between, and I would suggest that one's position can change as you go through life, as you have different experiences, as you have more life experience and engage with more people and as you learn. Examples of this might include women who become aware of sex discrimination when they become pregnant or become mothers and those who recognise the seriousness of sexual assault or domestic abuse when someone close to them is affected.
Interesting points from madwoman above. Sometimes, at least in my experience, if I find I'm struggling to get my head around another person's experience of marginalisation, discrimination, etc., I try and unpick it - and look at what from my own experience might be "blocking" my understanding of the situation. Is there a crucial bit of information that I'm missing and where can I get it? Am I concerned that advocating on behalf of that issue will have a negative impact on another, so am resisting somewhat? Is there something within my own experience that is "blocking" me from really getting to grips with the issue - perhaps my own hopes and fears?
Kind of on that last note, it can be a huge challenge to look at the abuse and disadvantage that women face just for being women (or Black people or Lesbian and gay people, or disabled people, etc. face for just being who THEY are,) and not in some way feel vulnerable. Their experiences remind us often painfully of the sorts of things that could happen to us as well (or even that may be already happening, but we aren't yet fully aware of it!)