I just ran across an old dump of HN from April 24, 2008. It contains the first 172,000 items (stories+comments) and the profiles of the first 6,500 users. http://rapidshare.com/files/3129266675/ycombinator-news-2008...

I would guess the secret sauce is spam filtering code. It seems conspicuously absent from Arc releases. The Arc code does come bundled with the HN source code, though, and all of the core features seem to work fine. There's nothing missing except some way of filtering spam.