Old Man's War is light and fun fare, even though it is about military warfare between humans and aliens, it never takes itself too seriously. The depth of intrigue and mystery is shallow. However, this book doesn't try to be anything more than it is. There isn't much description of the alien races or explanation of why things are happening in the universe as they are. Certainly, the main character is not all-knowing, and has no ability to discover this information, but without it, the book felt flimsy. It's almost like this is one plot-line of several that should be in one large tome.
The beginning reminded me of Ender's Game. It does make its own story, and the characters understand the implications of what they're doing, for the most part. The main character is rather boilerplate. Honest and loyal, strangely skillful and outlives everyone, with no discernible flaws other than he gets attached to others.
As another reviewer mentioned, I did pick up on the "whiteness" of the book. It stood out to me, mainly because of the name choices. I don't know if this was because Scalzi was simply choosing names that were retro, but it came off strange. He doesn't describe most of the characters as being a particular race, so it could be argued that there are a mix of people. However, it didn't read that way.