Now, this intrigues me. Maybe it's my general dislike of spoilers, but all I want to know before I read a fic is the general set-up and idea of where the story's going. Once in and deeply involved, I'm in the author's hands and I'll follow along quite happily. The two major instances of a major character getting killed off in a long fic that I was reading avidly struck me as quite justified in the story. The one of them by Marcus Rowland in The Rosenberg Inheritance hit me like a ton of bricks at the time, mainly because I wasn't expecting it at all. I think I left feedback at the time expressing the feel of I had no idea where the story was going from there, and that I loved the feeling of not knowing. (Marcus - If I didn't, then I apologise right now and I really should have)
The second major death was in a Harry Potter/Buffyverse crossover. I think it was The Muggle Way, but will have to check. At the time it happens in the fic, there's at least three main groups maneouvering around and investigating each other as the worlds start to collide and drift into each other. None of the groups have the complete information, and they're all making plans based on these flawed presumptions. So it comes that a raid on one location leads to a major HP character being killed more or less by accident. Now, this worked well for me as a realistic conquence of the mess that everyone was in at that point, and the violence inherent when the worlds clash. What also worked well for me was that the death had an effect on everyone else in the story - agendas were revealed, alliances drawn, all hoping to avoid things like that happening again.
Most of the main character deaths in fic that I've written have occured off-screen, and only for the effect on the characters that I am writing at that point. Like in Sleeper, Buffy's death is only there as a catalyst for the changes in Faith and Xander's relationship. Something like that, where it's not described beyond a few details and the character hasn't interacted that much with my main ones, I don't think deserves a character death warning. I'll be interested to know how you feel on this, considering another fic I've just written, but not posted yet.
I'm reminded of an Alan Moore quote:
"As I see it, a successful story of any kind should be almost like hypnosis: You fascinate the reader with your first sentence, draw them in further with your second sentence and have them in a mild trance by the third. Then, being careful not to wake them, you carry them away up the back alleys of your narrative and when they are hopelessly lost within the story, having surrendered themselves to it, you do them terrible violence with a softball bat and then lead them whimpering to the exit on the last page. Believe me, they'll thank you for it."
It's all about the journey.
So I guess what I want is non-shock value death when it occurs.
For it to mean something and for it to be not a regular thing - otherwise killing a major character would lose its power to shock and twist the story.