Yet another thought, crossing recent alt-fen threads:

So Draco comments that people who do Dark Arts "tend to lose their looks", and that Bellatrix doesn't appear to care about this. Dolohov says the new class is going to have a lot more theory and background to it.

It would be interesting to see his explanation for why, if Draco is correct, that might be the case. Because it doesn't seem to happen to everyone. (I mean, Lucius still has faaaaabulous hair....)