Characters (still spoilers here, folks.)
Ideally, in a proper story the death of a major character has a purpose, driving either the story or the development of another character.
One could make at least some case that Book died nobly, defending a colony. Can't see any good reason for killing off Wash. Zoe's character didn't change noticably. My daughter theorizes that it allows River to come into her own as a contributing pilot and character instead of oddball baggage. Maybe.