I was an angst-ridden teenager (and now I'm an angst-ridden adult, but nevermind...) when I first heard Joni Mitchell, and fell in love with her moody, highly personal songs. Songs like "The last time I saw Richard", "Ludwig's Tune", "Amelia", etc, paint really vivid pictures of a person or scene. Too many singer-songwriter "bad relationship" songs are neither well crafted, either melodically (think of the two chord rhythmic strum basis for many of them) or verbally (trite phrases, lines that don't paint a picture in your mind). I agree with Stephen's comment above:
What puts an introspective song over the top, as it were; pulls it out of the navel of the songwriter and into the hearts and minds of the audience is the potential for identification with the story being told. If the listeners can hear it and say something to effect of, "Been there. Done that." the work becomes a
communication and a sharing rather than merely a public whine.

Mitchell is a good example of a songwriter who I think crafts songs well, both in language and musical accompaniment, but with a few exceptions (Both Sides, Now and Woodstockcome to mind), her songs are written for HER voice. Other people's covers of them sound lame to me. One of the reasons that I and other traditional purist snobs say that Mitchell's and other singer-songwriters' material are NOT folk music is precisely for that reason. The writer may be crafting a perfectly wonderful song, expressing their emotions and personal history in a way that others can relate to and enjoy - but if their songs can't really be sung by other people, they aren't really ever likely to become "folk". Part of my personal definition of "folk song" is that it's a song that speaks to something inside a person enough that they are driven to learn it and pass it along to someone else. I've learned a lot of Joni Mitchell songs - but I sing them to myself, for myself, because they speak to me. I wouldn't be able to pass on the meaning in them to someone else in quite the same way that the author did to me.