Look at the distributions of the types of people that vote up the story, look at the differences in the average number of upvotes once it hits the front page, look at how many people flag it, but if you need to hard code things that legit users upvote you are basically imposing your view of what is good content over thousands of other people. This isn't a Viagra ad, this is a cryptography based, near-zero transaction fee, distributed, open source, online currency.
No wonder it always hits the front page, any of those five categories could hit the front page.
You just throw simple stats at the problem. You look at what a typical distribution of a normal submission looks like (30% of users < 60 days old, 20% of users < 100 karma, etc) and then you negatively weight submissions which breach the pattern by a certain sigma.
My bitcoin stories sometime got voted up to get to the front page for a brief time but they often drop out the front page fairly quick.
The current volatility of the bitcoin economy means that every single story on HN increases the value of their bitcoin "investment".