I've got to be honest: I wasn't expecting much from this book. I'd read the first book and enjoyed it, but you expect a book about a kid fighting off aliens to have a sequel with the same type of idea. Wow, was I wrong. This turned out to be what my have been my favorite "second book in a series" ever.
Speaker for the Dead brings huge new themes and topics into play; linguistics, biology, religion, and even the value of truth. It changes focus so drastically that I would almost describe it as switching genre. Speaker for the Dead isn't a story about fighting off aliens, it's a story about people and their motivations, why they went to kill the aliens and why they'd do it again, and the journey to accepting them as rational beings.
I was so surprised and pleased by this that I was planning to rate it four stars, the same as I had rated the first book, when I stumbled across a little snippet. Each chapter begins with an excerpt from some imaginary publication. Most were political commentaries, scientific reports, transcripts of conversations, or other such pieces relevant to the plot, and I enjoyed that. At a crucial point in the novel, though, it yielded a remarkable 400-word snippet from the imaginary "Letters to an Incipient Heretic." I won't spoil it for you, but those 400 words earned this book its fifth star. It was thought-provoking and put a well-known story under a different light I hadn't seen before.

To summarize, if you're looking for a book with battles against aliens like the first, this isn't it (although there seem to be signs that they might occur in later books). If you want a welcome break from all the expectations you have of sequels and to have a chance to look at everything from an entirely new perspective, this is the book for you.