Well, duh, Crabbit fool. Surely a debut is just a first book, different from your second only by virtue of being first. No, not necessarily so. And if you think like that, you may never write one. Or rather, it may never be published, which makes it not a debut.
I recently wrote a blog post for Catherine Ryan Howard about debuts - on the subject of Mondays are Red having been my debut, although also not my debut. Do head over and read it and then come back and ask me a question or say what you think.
I think it may help you in your struggle with that awful agent response, "I like it but I don't love it enough." This phrase doesn't necessarily mean that your writing isn't good enough or that there's anything you can do to that novel to make it lovable-enough. It may "just" not feel like an idea or execution which is going to set the sales and marketing team alight sufficiently for them to take it.
Which, frankly, is very gah-inducing.