Death is one of those things that fantasy gaming and literature is always getting wrong. It should matter, it should be the end off, or at least a defining moment in a characters story arc.
In the words of Max Landis, "the death and return of superman, forever broke death in comics". Once you trivially return a major character to life, you can trivially return all characters.
Fantasy roleplaying games, especially those in those forms closest to DnD are generally less that respectful of death. In DnD it is, even for relatively low level characters a trivial thing to return form the dead. Death is also often a relatively boring part of the game, an obstacle to overcome with a bit of money or a spell, or at best, a sources of cool monsters in the form of those that refuse to stay dead.
So how do we get back to a point where death is meaningful once again? Well one possibility, and probably the most important, is to remove magics that allow characters to be brought back from the dead. Being able to return PCs to life is perhaps the single biggest contributor to this issue. Another possibility is to take a leaf from Greek myth, where the underworld is a place which is fairly active. Heroes visit it, are dipped in the waters at its boarder, and rescue loved ones from it's clutches. Being able to explore the world of the dead, and gather from it treasures and knowledge means that it is a living breathing place, which will matter more to the characters and their players.
So, next stop, the shadowfell...