Old Man's War

You can find a more detailed review of this over on sffbookreview
When I read this, I enjoyed every page and would not put the book down. However, there was one thing that bothered me while reading and bothers me even more now, a while after I've read this.
The side characters, in fact the entire group of “Old Farts” are cardboard cutouts only there to explain things to our protagonist and maybe die to show us how brutal this war can be. I didn't have time to get attached to them, I couldn't even really tell them apart.
Scalzi may be a great writer who know's how to keep my attention and make me want to read more, but this story? Very bad on the character side (and that's what I value the most).
Plot, story-telling and world-building: Absolutely brilliant.