You're onto something. Team-BHP [1] is run exactly like this, and it seems to be working.
For those wondering, it's a car-enthusiasts website based in India. They've been around for around 18 odd years I think.
The moderators all have actual dayjobs.
When signing up you have to write a paragraph about why you're really a petrolhead (or dieselhead because Indians love European turbo-diesels :) ), and there's a human on the other end vetting your sign-up application! Plenty, including me, have been rejected atleast once. I got in on my 2nd attempt years later.
As a matter of principle they refuse to do car advertisements.
I don't know how well the site is engineered but it works. Check it out. But I suspect most non-Indians (such as most people on HN) wouldn't find it that useful as it's mostly about the Indian car scene.