I've been reading (and listening to Goofy read) several posts by Snape lovers about how JKR shouldn't have killed off Snape. I'm going to disagree; I love Snape as a character and I mourn his passing, but JKR has in no way "shorted" Severus Snape.
Severus Snape is one of JKR's greatest achievements in this series, perhaps the greatest. He is clearly the bravest man within the series, the most tragic, and one of the most heroic. In Severus Snape, JKR has created a man who is not nice, is not kind, is not at all likable, but is truly good and truly heroic. Let's look at the facts:
Bravest: Yes, Severus Snape is the bravest character within the Harry Potter series. It isn't that Harry isn't brave, he is, but Snape is the braver. He has spent years doing what must be done to keep the Wizarding world safe from Voldemort and his Death Eaters. As an example, he ran Hogwarts for Voldemort for nearly a year; however, although many of the children may have been hurt, he kept them alive with the ability to heal.
Most tragic: This one is easiest; Severus Snape never got a break in his entire life. Although in some cases this was due to his choices, it was largely due to circumstances outside his control. A poor boy who wants better for himself--of course, he would be sorted into Slytherin. From there, the rest of his life was one tragedy after another. And yet, he never stopped loving Lily, and spent almost his entire adult life protecting her son, in whom he could rarely see other than his hated school/romantic rival.
Heroic: Although I would say that Harry is the most heroic character of the series, Severus Snape is Number Two. He fought Voldemort while acting in such a way that everyone believed he was on Voldemort's side. He killed the one man who truly understood and believed in him, an act which he clearly did not want to do, on that man's request. The only reward he asked for was, at the end of his life, to be able to look into the eyes he had loved since he was a child.
Severus Snape was JKR's demonstration that one can be good without being nice, kind, or likeable. He shows that one can be on the same side without being friends. He is the bravest man in the series, a hero who will, I hope, be greatly honored in the Wizarding world he worked so hard to save.
R.I.P. Severus Snape. I hope you get to spend quality time with Lily in the afterlife, and that your afterlife can be better than your life.