>ForHackernews, we say in our quality guidelines to "Make pages primarily for users, not for search engines."
The problem with that is that it's not true (in my admittedly very limited experience). I used to believe that until I created a small website and found it wasn't getting listed in Google and I had to try and guess why and carefully read the rules.
Making pages primarily for users means you unthinkingly do things like having duplicate content on multiple pages/subdomains/sites or linking to a site that reviewed your product after you gave them a free sample (a paid link).
I'm sure to any even vaguely experienced webdev they know not to do these kinds of things (use canonical links, nofollow and all that jazz). Somebody just throwing a few pages together who has no experience in this stuff easily falls foul of the rules. The rules may seem obvious to people who understand how search engines work. If you have never heard of pagerank and don't have an understanding of how search engines work and how people try to game and spam search engines, the rules are not obvious at all and certainly the rules are not the same as the natural behaviour of somebody creating a legitimate site with the users in mind.