How we cite our quotes: All quotations are from Harry Potter and the Sorcerer's Stone.
Quote #1
OLLIVANDER: I remember every wand I've ever sold, Mr. Potter. It so happens that the Phoenix whose tail feather resides in your wand gave another feather. Just one other. It is curious that you should be destined for this wand when its brother gave you that scar.
HARRY: Who owned that wand?
OLLIVANDER: We do not speak his name. The wand chooses the wizard, Mr. Potter. It's not always clear why, but I think it is clear that we can expect great things from you. After all, He-Who-Must-Not-Be-Named did great things. Terrible, yes, but great.
Ollivander seems to be implying that good and evil are not so far apart here, or at least that they're connected in ways the Harry might not be entirely comfortable with.
Quote #2
HAGRID: First, and understand this Harry 'cause it's very important. Not all wizards are good. Some of them go bad. A few years ago there was one wizard who went as bad as you can go. His name was V--. His name was V--.
HARRY: Maybe if you wrote it down?
HAGRID: Nah, I can't spell it. All right, Voldemort.
It seems silly, but there's a literary precedent to this. In early Christian stories, calling the devil by name could summon him. In modern urban legends, we have the "Blood Mary" story, where you're not supposed to look in the mirror and say her name five times. Voldemort, like those other figures, is so evil that even breathing his name will get you into trouble.
Quote #3
HARRY: Me? Voldemort tried to kill...me?
HAGRID: Yes. That ain't no ordinary cut on your forehead, Harry. A mark like that only comes from being touched by a curse...and an evil curse at that.
The scar makes a potent symbol; not only of Harry's ability to survive a great evil, but the fact that survival doesn't leave you untouched, and that he'll see the price he paid every day when he looks in the mirror.