One thing that is different here is that this site does not need to generate (ad/any) revenue, so many of the irritating things that will be common in other forums will be absent here.
I'm pretty sure that eventually many more really successful websites will find a way to adopt a model like that, it really is one of the best ways to marry a happy users to commerce, find some side-channel that you can exploit.
Most websites think that the 'data' they have on their users is the marketable good, google has often been said to see you, their user as the end-product. YC does it different, they see a subset of the users as the raw material to drive their bottom line, the rest are along for the ride.
It's a much more direct version of what FB, google and so on do, and it is much better imo because it is more honest and transparent.
It's hard to put the difference any clearer than that, but YC engages a small subset of their users in activities that they are peripherally engaged in on the forum. Other 'social' sites sell their users data wholesale, that's a completely different kettle of fish.
Most of the users on HN are not engaged in the revenue stream of YC, directly or indirectly whereas almost all the users of other social sites are directly involved in the revenue stream.
I can assure you, in publicity and marketing circles, when social media is mentioned it is precisely to attract consumers for your product.
The type of interaction between HN and YC is not new. It's just hard to replicate. Companies are trying this since the dawn of the internet. Think about what is the forum on the World of Warcraft website. The social media craze is an attempt to apply this community feeling to other markets.