Just a thought about Voldermort...I think I've figured out something really important to the story line.
In reading up a bit on JKR's web site, I came across something really important. She said that the questions people should be asking aren't "why did Harry live?" but instead, "why didn't Voldermort die?" I reread the first book just recently, and stumbled upon something of great importance:
"...not being truely alive, he cannot be killed."
This refers to Voldermort in his bodiless state, but I really think it has more to do with it than that. I mean, it would make sense, wouldn't it? Voldermort was once Tom Riddle, and went through a transformation to become the Dark Lord. He must have sacrificed something to gain so much power...