The point that Seth makes is that for most projects, you have to be able to be able to categorize it. People don't want to know what it is, but what it's like. That gives them a quick, experiential point of reference they can build on. "It's like King Kong, but with a giant bunny" lets the audience's memory/perception quickly fill in a whole bunch of blanks so you don't have to.
On a rare occasion, though, your project is so unique, it defies a simple categorization. You can't get that quick hit of familiarity. You either have to describe it in full, without the aid of familiar references, or you have to say "it's like nothing you've ever had. Just trust me and try it."
People would ask me to describe my novel in just a few words, and I couldn't. It wasn't a "vampire novel" per se. It just happened to have a vampire in it. Some of the story was driven by Alain's vampirism, but a lot of it wasn't. There was a lot of borrowing from Dante, some from Milton... Categorizing it was very difficult for me. I was too close to it to be able to boil it down to a few catchphrases and keywords.
I didn't know if it was that unique, or if I just didn't want to categorize it. To categorize it feels like you've not only limited it, but you've taken away a degree of its uniqueness. So, as the creator of an "artistic" work, it's quite possible I was merely resisting categorizing my story rather than the story itself resisting categorization. Every child is unique, right? Even if they aren't.
But when you're trying to sell a work, saying "just trust me and try it" is not a great approach if you haven't built trust with the person. Furthermore, when you say "this is unlike anything else," you have to be 100% sure it is unlike anything else. If someone gives you the benefit of the doubt, reads it, and says "this is just like...", you're screwed. You asked them to trust you about it's uniqueness and lost.
I'm still on the fence over whether my novel is resisting categorization or I'm resisting categorizing it, but Godin has given me some insight that is helping me look at it more honestly. If I want to sell it, I need the best answer for "what's it like" that I can find.