The biggest problem is that job descriptions sound like they were written by HR. Sometimes there are numerous qualifications listed which are unnecessary for the position.

Plug: If you want help writing a better job description, we're one of the companies quoted in the article and can help you (HireArt). We do it for free if you post a job on our site. It's pretty crazy how much of a difference it can actually make. If you do it yourself, my biggest advice is: make it memorable. Funny, sweet, crazy, whatever...but make it something people think of even after they close the browser window. www.hireart.com

I have to agree with the comment from johnjlocke. Also, many job descriptions focus too much on providing a laundry list of requirements and don't touch on what the role can mean for the candidate (how they can help the company achieve its larger goals and how it can help the right candidate do challenging things they love). Plug: We have a guide on our site gleaned from experience helping to recruit and hire tech roles for our portfolio companies: http://labs.openviewpartners.com/boring-job-descriptions/

Also, here's a list of three examples of great job descriptions from TLNT you can check out: http://www.tlnt.com/2012/08/02/three-awesome-examples-of-gre...

Joblint (https://github.com/rowanmanning/joblint) points out "sexism, culture, expectations, and recruiter fails" in job descriptions. Pretty clever.

I saw some job postings on HN earlier that didn't talk about what the company does. This is key!