Frequently on HN we get a lot of people who are great at building stuff but fall when it comes to finding and growing a userbase.
We then have people that are great at developing a community, have a popular niche blog or forum, and are itching to monetize their community.
Serving ads to solve this problem is like strip-mining for gold - ridiculous sediment to gold ratio.
So what if we concentrated on the affiliate model. Rather than serving ads to your community, you vet actual products and services that your community would actually use and genuinely endorse the product openly. You could run a drop-in type of "marketplace page" for your endorsements. It would be clear to your community that you receive (large) commissions on these sales, but I reason happy users would want to help your site. I'm talking 50%+ commissions so you can easily profit share in terms of offering a better price than they'd pay going directly anyway.
For product and service providers, I can't see a downside. They pay only from actual sales. The product is seen in a positive light (endorsed) rather than overlooked as an ad. And product providers could/should work closely to community moderators to iterate on their product.
As of now this seems like wishful thinking I know, but this could definitely be worked out, and provided it was dead-easy to get started from both ends, it's worth testing.
Make dropping in an affiliate marketplace easy for content providers. And make revenue sharing, management, and tracking easy for product providers. Develop community around connecting the two in a vetted environment. Profit off transaction fees.