Well, there isn't always a good plausible reason why people die- especially in wartime. Or in my shadowrun group, where the morons tend to outnumber the smart runners, and the morons are the ones that somehow always find a way to survive.
Book's death was sad for me, because I really liked the character, but it seemed like I was in the minority there. Wash's death upset a great number of people, because we ALL really liked Wash, but it also gave the impression that anyone could die.
You might not like it, but it is a great storytelling device- Whedon took an established hollywood law (you can't kill the important or likeable characters), and broke it thouroughly. Wash was probably the most likeable character on the crew, and his death was a shock. If Wash could die, then anyone on the crew could die- this wasn't the tightknit group of heroes who remain unchanged per Stan Lee's 1970 instruction that main characters only undergo the "apprearance of change." They were capable of dying, and it made Mal and The Operative's fight that much more tense, and it made the rest of the crew standing off against the reavers that much more threatening and scary. And it made River's sacrifice that much more moving- she really could have died.
I liked it, and I thought it was good writing, even if it didn't follow the established laws of literature and film.
I could compare it to Harry Potter and the "someone is going to die this book," thing JK Rowlings does, but I know Mr. Potter irritates some people here.
-kv