I was flicking through tracks I've imported to iTunes (vinyl and CDs), and just listening to the intro, and to be honest, I think in many ways the opening that stops you in your tracks is pretty much the essence of Northern. And not just Northern Soul, Funk, Deep, and probably many other genres too. I think the ones that appeal to me are the ones which are quite off-beat, so to speak.
I don't think Mr Flood's Party has been listed yet for instance, or Eddie Floyd "That's All". I love the tunes where there's some sort of odd chord, not knowing music enough to describe it, but it is some sort of flat maybe.