Because to me his death is the most pointless ever. He was such an interesting character, with a great background and a story of guilt, redemption and second chances, and instead of using all of this Rowling killed him in the most lame way, like "oups, he fell through the curtain of death, too bad!". Lack of a good plot and story writing skills = plot hole (but that applies to a lot of things in book 5 in my opinion)
yep, the boy who lived is also the boy who make you not to live!!!