The weird thing about the death of characters in Lost is that while dead characters make as many comebacks as ghosts, dreams and visions as dead characters did in Buffy their death feels much more permanent. When you get intimations that a character like Jin or Claire has died there is a sense of loss (relative of course, this is still television).
I think that’s because when a character dies in Lost they don’t actually return as the character you originally emotionally invested in. The involved storyline involving the character actually comes to an end.
You also have comparatively minor characters like Boon that seem to get sacrificed or punished as a consequence of the actions of the main characters. The surprising thing about these deaths is that they often occur before the character’s narrative arc is complete.
That might simply be due to real world factors about the actors and the rigours of shooting big American serials but an interrupted story arc means that the viewer has a sense of a story being interrupted prematurely.