On the topic of character death, I can't help but think that Seven's death instead of Janeway's would have produced a similar, if perhaps slightly less vocal, reaction, as there have always seemed to be more Janeway fans here than Seven fans. (For the record, I like both.)
Regarding the idea of killing a main character, I think I understand why it was done (and no, I haven't read the novel yet and probably indeed won't) but I think, perhaps, the way it was presented (again, without reading the novel) might have been part of the issue. I think the writers needed a universe-shaking event to maintain interest and drama in the novels, and also didn't quite know what to do with Admiral Janeway. But, also, her sacrifice appears to have been just that - a sacrifice as a part of the dynamics of the story that was not quite presented as such.
Indulging in a related analogy, as many of you may or may not know, I'm more than a little fond of Kirk's son, Dr. David Marcus, from TWOK/TSFS and the late actor who played him. Obviously, his character (and the original Enterprise, which I consider a character as much as anyone) were both "killed" in TSFS. Producer Harve Bennett (and many storytellers) have a notion of interstory karma where the protagonist has to lose something to win. In TWOK, Kirk lost Spock to escape Khan. In TSFS to "regain" Spock, Kirk had to essentially cheat nature and so had to lose both his son and his ship.
Now, I realize that David Marcus was not a main character per se, but I think that the comparison between his death as "karmically necessary" and what I see as necessary foundation for a Seven or Janeway death is still there. It seems like in the book, such a foundation wasn't really expanded upon and therefore many people feel it was more stunt than part of an organic narrative.