Kestastrophe - The book will explain, in considerable anatomic detail, why these tweaks make a difference.
I noticed a fairly substantial psychological effect as well. I go down into the squat much faster when I'm not looking at myself, because I'm not using the mirror as a gauge of squat depth, consciously or not. Also, it helps shift your mental focus from the bar (which you can't help looking at if there's a mirror there) to your feet and ass (which is where the weight should be distributed, and what moves the weight, respectively).
Draft - Didn't hear about the Lyle McDonald flap. I read his posts occasionally but he's a little bit too hardcore technical with his diet for me, to be honest. I'm just not ready for that level of detail. Robb Wolf is more my speed (and that guy can LIFT too). Even then it's too much information.