One of the small things I liked about the movie was, when they were preparing to hold off the Reavers, Jayne and everyone else are all busy moving crates, etc. Zoe is just standing there the whole time, staring at the door and loading her shotgun. Nice contrast, with her as the motionless center of the room and everything else all motion around her.
I also liked the Big Damn Tracking Shot near the start of the movie to introduce the ship and everyone aboard. That was pretty impressive, as long, continuous shots like that are hard to do and require lots of rehearsal. What made this one even more difficult is periodically, the cast and camera had to jolt about as if the ship were encountering turbulence.
As for Book and Walsh, and the topic of sequels: The first time I saw the movie was at the first batch of advance screenings last May in Austin. Ron Glass and Nathan Fillion were at the screening. Afterwards, Ron said both Joss and Nathan had talked to him during filming to say that Book's death in this movie did not mean Book would not be in future movies. There is the possibility of prequels (or at least movies set during the time of the series, or between the end of the series and the movie), or flashbacks, or other things. Ron also said he would be in favor of that, depending on the script.
Ron also said, when asked by someone about Book's past, that the only thing he would reveal is that Book was not a virgin.
As for Walsh, Nathan basically said that Alan was too much in demand to be able to commit to future projects. Alan had more credits than any of the others -- except Adam Baldwin -- prior to the series (28 Days, A Knight's Tale, Patch Adams, Wonder Boys, Hearts in Atlantis) and afterwards (Dodgeball, I Robot, Into the West). He's an in-demand character actor with a growing portfolio. I also think Nathan didn't like having him around on the set because Alan is funnier than Nathan.
Like many others, I thought Walsh's death signified a TPK. I thought Joss was going for a Hamlet ending -- everyone on stage is dead. Then when River jumped into the room full of Reavers, I felt she would survive.