On the Richard & Judy show, JK said this months ago:
JK: Yeah, one character got a reprieve.
R: Oh really?
JK: Yeah.
J: I mean you are, I just...
JK: But I have to say two die that I didn't intend to die.
Having read the Deathly Hallows now, the whole discussion about this bit of insight from her gained extra footing. But what did the deaths bring from the perspective of the whole story.
Charity Burbage - gets the point across. The war has started, and it is real.
Hedwig - gets a now-useless distraction out of the way. Harry couldn't very well carry around a large cage with him without JK acknowledging the fact that they have to take care of that too. How would Hedwig fare in Hermione's beaded bag? Ron's Pigwidgeon disappears from the story too after The Burrow. Two reasons, why she could have pulled that off. First, it is Harry's story, not Ron's, so the readers don't have to follow Ron around feeding his pet. Second, Pig is native in Britain and would attract no attention compared to the white, arctic, diurnal species and could fly around and find them on his own in case of a rushed escape. Similarly, as far as I noticed, Crookshanks is mentioned only at the Whomping Willow too. Distractions.
Mad-Eye - of course.
Dobby - Harry's mourning was excellent. Pure bliss to read. I relished it with heavy tears streaming down my face. The Free Elf is the only one in the whole book who dies a heroic death. Although I disliked the simplicity of Dobby's actions throughout the series and found him rather annoying, the choice to give the heroic death to him was a wise one. JK erected a monument for the downtrodden and beaten. Any wizard dying a heroic death would become a martyr in the eyes of public, raised onto a pedestal and worshiped. In choosing an elf JK was able to beat down the proud wizards a notch without being too obvious about it.
Fred - too many Weasleys, frankly. It is still a miracle that only one of them died. Moreover, Fred in his second best scene (Potterwatch!) acknowledged and openly forgave Percy. But killing off a twin? As a mother of twins, I must say what I say often in RL - the fact that we have two instead of one does not mean that we have a spare one!
Lupins - casualties of war. If Voldemort said the truth at the end, and the Hogwartians lost half of their troops, only three Order members and no teachers we were shown dead are still painfully underrepresented.
Colin Creevey - casualty exhibiting the fact that not only Marks Evanses and the main stars are targeted, but also the "extras".
Bellatrix - Molly's 15 seconds of fame. Brilliant. Can't you just imagine a short, plump witch in homely robes battling a tall, distinguished, excellently dressed caricature of Voldemort's right hand? All those hours spent managing the kitchen at The Burrow with the perfect timing of flying knives and pans and constant threat of F&G distractions proved to be an excellent training. I liked that duel almost as much as McGonagall/Snape's transfiguration lesson.
Nagini - it is great that Neville went after Nagini, not Bellatrix as so many expected. He did wonderful. I just hope that off-stage, Harry returned the sword to goblins.
Voldemort - defeated by his own arrogance. Nice.
In summary, I enjoyed the deaths. I cried my heart out for Dobby and still weep for Severus. The book was not nearly as bloody as I expected. And the three characters that changed their fate? Lupin and Tonks, in my opinion. More Order members simply needed to die, and going after "extras" would not get the point across. Also, it couldn't all have been the Weasleys. The reprieved one would be Draco then. Perhaps JK felt that Narcissa deserved to find him alive after covering for Harry in the Forrest. Perhaps she felt that he deserved to learn his lesson that started when he lowered his wand at the Tower. Originally, I expected one of the Trio to get the famous reprieve, but seeing as our magnificent Jo decided to manage the plot, Harry did not need additional anger, grief, love, whatever to defeat Voldemort. As such, death of Ron or Hermione would have been unnecessarily painful in a children's book.
I can't be more sorry for Severus though. Appart from my utterly selfish feelings of a SS/HG "(wor)shipper", he deserved to live on more than anyone merely for the 20 years of living on the edge for one decision of the Sorting Hat. But if he could not get over his love for Lily, maybe it's better he is dead. I want him to be happy. If it takes for him to die to achieve that, so be it.