I think it would be nice to have a visual distinction between unfriendly and friendly monsters, such as wraiths.

I agree. I've been waffling around with how to do this well, though. In tile mode, it's pretty simple; add a border, or a small icon, or colorshift, or what-have-you. ASCII mode is harder. I'm not a big fan of the NetHack method of coloring the background, I think that tends to stick out rather unpleasantly against the rest of the screen.