|Credit: Better Book Titles, Title and Redesign by Lauren Dee|
But above all, as the above image from Better Book Titles indicates, it may almost be known for Martin's unflinching unsentimentality about his characters.
When people warn you not to get attached to Martin's characters, they usually are referring to the fact that (mild spoiler) people die that you probably don't expect would die. Martin shows a remarkable capacity for killing off characters. Don't get attached. Anyone may die at any moment. And Martin uses this atmosphere to terrific effect.
But I'd argue that Martin's unsentimentality goes further, and there's a lesson there for all writers: Martin lets his characters have flaws.
Every character in A Game of Thrones has a set of positive and negative qualities. And Martin is not afraid to go dark. Even arguably the most noble character in the book, Eddard Stark, has somewhat of an inglorious past. He suffers from hubris. He is on the whole a good person, but he's flawed.
We writers can get really, really attached to our characters. They become almost like family members. We want the best of them. And sometimes it becomes difficult to see them make mistakes and to see their flaws and to let those bad qualities shine through from time time. We can be far too nice to them.
Martin has no such compunctions. He isn't afraid to show the warts, to revel in them even, and to trust that the readers will still feel affection for the characters. If anything, seeing characters with flaws makes them feel more human.
Don't be afraid to uglify your characters. And, as the spoof book cover says, don't get too attached.