Well... as for the entire issue of whether or not to kill of your characters, for me, it all depends on the story. When I write, I just write the story as it comes to me. The characters and plot both sort of take on a life of their own, and I let the character's emotions and motivations dictate what happens. I won't kill someone just to kill somebody off and make my story more "realistic", but if my story necessitates that particular character dying, then I have no option- I need to kill him off. An example from my own work:
Charlotte in The Wolf's Rebellion. The scene I had in my mind- Prince is with her, Sir Evan goes to collect him to bring him to the King. Writing it, I had no idea I would kill her off. Then, I thought about Sir Evan's personality and his responsibilities to the King- he'd have to kill her. If I did anything else, I would not be doing his character justice, and that would against my belief that when you're writing, you always have to be honest. No matter where the story takes you, as long as you're staying true to your characters, that's where you need to go.
I don't believe it determines whether a story is good or not, though. It all depends on the story- some stories, in order to be the best that they can, need a lot of characters to die. Other stories, on the other hand, need a lot of characters to live.
Oh, and Firefly- Johan Liebert rocks.