The density thing is kinda easy to figure out - less air space around the contact points. Kinda like how food in boxes stores in the cupboard with less wasted space than food in cans.

So that means more mass per length than a same-gauge roundwound string ... which suggests to me
  • darker tone - clearly demonstrable and already mentioned
  • more sustain
??
hmmmmmmm..... do groundwound strings have more sustain?

I'm wondering if there's a flaw in my logic on that one, because by that reasoning flat wound strings should have yet more sustain due to their even higher density, and I dont' think of lots of sustain when I think of flatwounds. Maybe I'm wrong there, too, as I've never put a new set of flatwounds on a guitar for comparison. the only time I've ever played flats is by picking up somone else's (unfamiliar) guitar.
Doctor, I have a constant ringing in my right ear.