No, don't do that. I was a literary agent for 14 years, and questions like that are the reason I stopped (in most cases) giving personal feedback on rejections. The answer is not going to be a word or two. If the book simply isn't at the necessary level of excellence, there's no way to explain that without going into an entire lesson in writing. It's not agents' business to teach writers to write, any more than it's writers' business to tell agents how to sell.

I understand that you're starving for serious feedback. That's true of most writers, until they start selling their work. But it's hard to start selling without that feedback: a perfect Catch 22, and the precise reason I started doing these reads for writers. Give me a shout if you'd like to take me up on it.