"...everything that was good about this story lay in the relationship between [the angel] and [the demon]."
My problem is that I, personally, do not care as much about the human actors in these stories as I care about the angels and demons. This might not be (much of) a problem in a short story, but it's a huge problem in a novel-length work. I can skate over side characters' characterization and motivation and all that happy horse poo in a short, because the story isn't about them. In a novel, not so much, because the sheer scope of a novel means that you must delve into those things--especially when the "side" characters aren't, not really.
I've got two humans running around in Hitman in Hell. They have their own hopes and dreams and wishes in this thing, and I really need to figure out exactly what those are, besides the obvious "get out of Hell" ones. The Hitman knows he's not getting out because he deserves to be there (so what does he want besides that?), and the Girl wants out and has an actual chance of making it (she's there because of an egregious clerical error) if they don't foul it up too badly (so what does she want besides that?).
The angel and the demon have a History, and so that's the Fun Character Stuff with them. They're easy. The people, not so much.
I've kind of got the same problem with Angry Bitter Angel. There's four humans running around it in (the Serial Killer and his Three Victims), and I really really need to make the reader actually care if the Serial Killer turns his life around, on his own account rather than his Guardian Angel's account. I also need to not rely on the reader filling in the pathos of his victims based simply on the fact that they're victims. What do they want (besides not to die)?
Gah. Novels are hard.