Today's Harry Potter Topic of Great Significance is...hair.
JKR spends a fair amount of time on Harry's hair, particularly on how unruly it is. Once, when Petunia cuts it all off, it grows back overnight. Hermione's "bushy" hair is similarly hard to control, except when she uses immense amounts of Sleekeazy's Hair Potion on it. Snape's hair is perpetually greasy. I read a fic that suggested that his hair was greasy no matter how much he washed it.
So maybe these people just have problem hair. But how boring! Much more interesting to speculate that wizards with strong magical ability are always subconsciously affecting their own appearance.
So, Snape's greasy hair is due to his subconscious making him look as he thinks he deserves, or possibly trying to look unattractive so that no one will want to engage him.
Harry has a subconscious memory of his father's artfully dishevelled locks and wants to be him.
Hermione...well, maybe Hermione just has problem hair.
Also, JKR puts quite a lot of emphasis on hair colour. I find it most interesting that the Dursleys and the Malfoys are all blonds. (I don't think Vernon is definitely stated to be blond, but his resemblance to Dudley, who *is* blond, indicates that he is.)
Riddle, of course, has "jet-black" hair, just like Harry.
The Weasleys and their red hair have got to be significant somehow. I'd like to read a study of redheads in fiction. Are they usually sidekicks? Tricksters? Villains?
You know what would be cool? A character guide to HP that had little coloured boxes for everyone's hair colours. Then we could more easily engage in this sort of vital in-depth analysis.