Well, after pestering him via email, I heard back from the nefarious agent who'd promised feedback. I have to say, his response was still pretty unsatisfactory.
Basically, he said it's a matter of opinion. There's nothing actually "wrong" with my work. No "grammatical or spelling errors, awkward sentences, or significant structure and composition problems" to blame it on.
He then went on to compare me to Beyonce, in explaining that some people like her music and some don't and it's subjective. Keith liked this, because it made him Jay-Z, but I'm still not satisfied.
He ended by stating that, while "there are no blatant problems with your book, it just did not work for me...thus, I am sure another agent will love it and make a great sale."
I can't decide if this is sincere, or I'm just getting blown off. Either way, no usable feedback! Maybe next time.