EW.com posted some answers to fan questions from George R. R. Martin:
Why do you find it so easy to kill off your main characters? —Brian H.
It’s really irritating when you open a book, and 10 pages into it you know that the hero you met on page one or two is gonna come through unscathed, because he’s the hero. This is completely unreal, and I don’t like it. If I was a soldier going to war, I’d be pretty scared the night before a battle. It’s a scary thing. And I want my readers to feel that fear as they turn the page. I want them to feel that no one is safe — that if my character is surrounded by three people with swords, he’s in serious trouble, because he’s only one guy against three. It’s a great way to show that you’re not writing this cartoon adventure where the hero is going to slay 20 men at once with his brilliant swordsmanship and go through unscathed while making wisecracks all the way.
Strangely enough, I discovered this link in the "related pages" sidebar of my gmail account. Hey, those things are actually good for something! Who knew?