We tried placing ads for ninjas, rock stars, and so on, but I discovered this was the cultural equivalent of advertising for white males who drink dry martinis. Not that white males who drink dry martinis can’t do the job, but there’s no real difference between advertising for a Ninja and throwing half your resumés away because you don’t like unlucky people. Either way, you end up with fewer resumés.”
This is so true, so important, and so many startups (and even bigger companies!) miss this. Job ads provide cues, conscious and subconscious, to the people reading them. Not everyone reading the ad is identical to the person writing it, and a badly written job ad can easily send the message "this company isn't for you" to a large number of skilled potential applicants. This applies not just to categories like gender or race, but even to personality types and personal interests. Unless you really want a company of only extroverts, for example, don't write a job ad that scares off introverts.
In the canonical example, if you constantly ask for "rock stars", you will turn off people to whom that doesn't appeal, including tons of good programmers. But it goes beyond that: don't assume that all your applicants are any particular kind of person with certain interests. A job ad should focus on what the job actually is, and things that are important to the job.
The best programmers often have a lot of choice in where they work, and as many HNers know from experience, if they see a job ad that turns them off in some fashion, they will probably not even bother reading further: they know they have better options, so yours probably isn't worth their time. If the vast majority of skilled programmers skip over your resume, it's no wonder you only receive resumes from unqualified applicants.
In short, when writing a job ad, you need to think from the perspective of people applying. Use your empathy, put yourself in their shoes, rather than just writing what you think looks cool.