This essay, entitled "The problem with asking around 'Is my story idea any good?'", is a good, succinct treatment of a common problem.
It doesn’t matter if your story idea is good or bad. An idea is just a spark of motivation to start writing. ... There is no reading experience in an idea, only in its execution.
Emphasis mine. My theory as to why writers demand a pre-emptive answer for whether or not a given idea is any "good" is because they're trying to avoid a sunk-cost situation. They don't want to invest a year of their time in something that's a known dud.
There's other impulses behind this, too. For one, it's far easier to ask someone, "Is this a good idea?", and to work from the answer you get, than to explain how you plan to treat the idea and where to take it. It ain't where you're (coming) from, it's where you're (aiming) at.
Over the last few decades we've become very good — as audiences, not just as creators — at abstracting the concept away from the execution of something even before it's executed. If I say that a given movie concept is Pretty Woman meets Out Of Africa (ecch), you may not like the idea but you can form a pretty good image in your head of what it might be — not just based on those two reference points, but all the ways previous projects fused things in roughly the same way.
I wonder also if that collective cultural memory for how ideas can be combined and run down makes us seek conceptual approval all the more. We feel all the more obliged to make something that reminds people of the last several other things that got mashed together, or which seems like an appealing mashing-together of things based on the way other things have been mashed together. When there's just so much out there to experience — not just read but watch, listen to, play, etc. — any advantage you can wring out over the competition is worthwhile, right?
But the only way to know what you really get from such work is to go do it. There may not be a way to know ahead of time. Doubly so if you're lighting out for totally uncharted territory.
Something else touched on in the original piece: What drew you to this idea in the first place? What are you thinking of trying to get out of it? Few of us really seem to confront why we are drawn to a particular something, maybe because we feel we're writers and not critics, and that kind of analysis is best left to someone else. But it's a practice worth cultivating for our own sake.