I haven't seen the first thread but I think Alec has got the flavour here: songs which make you uneasy because you feel it is inappropriate subject matter for song as a vehicle, or songs where the writer deals with human instincts usually controlled in the adult psyche. Again, there are many trad songs which have bits that make you feel a bit queasy: Will o' the Winsbury where the father says he'd fancy Will if he were a woman; the song quoted about the dog reminds me (is it the same?) of the ballad Carthy sings about the poisoning of the son - quite gotesque.
   Also, too confessional a song can be difficult: Lennon has a few.
As for ones that simply are unbearably moving, I'd add Andy Irvine's "West Coast of Clare", Jimmy Webb's "Sandy Cove", "Mary Morison" and "Beeswing".