Having no regard for the candidate's time. From last-minute cancellations, without apology or acknowledgement of the inconvenience, to not paying attention in the interview, some employers act like their time is the only time that matters. Most candidates go to a lot of trouble to prepare for an interview -- reading up on the company, taking time off work, and often traveling--and their time should be respected too.
Not sharing their timeline. Employers have some idea of whether they'll be getting back to candidates in a week or a month. There's no reason not to share that information, and it can be agonizing on the job seeker's side to have no sense of the timeline the employer will be moving on -- and yet many employers keep job seekers uninformed.
Refusing to share their salary range, but asking you for yours. Employers know roughly how much they're willing to pay; there's no reason not to share that info, other than that they're hoping to get you for a lower price. But that's lame: If they lowball you now and you figure out later that you're underpriced for the market, they risk losing you over it. They should tell you the range they expect to pay and put an end to all the drama and coyness.
Misrepresenting the work. Interviewers who make the job sound more glamorous or downplay less attractive aspects of the job--such as long hours--are guaranteeing they'll end up with a bitter employee. Truth in advertising works to everyone's advantage, because candidates who won't thrive in the job, or the culture, can self-select out before they become your disgruntled employees.
Not notifying candidates that they're no longer under consideration. This is both common and inexcusably rude. Candidate are often anxiously waiting to hear an answer--any answer--and end up waiting and waiting, long after a decision has been made. It's about simple respect and courtesy (and it just doesn't take that long to email a form letter).
Alison Green is the author of Managing to Change the World: The Nonprofit Leader's Guide to Getting Results. She is chief of staff for the Marijuana Policy Project, a nonprofit lobbying organization, where she oversees day-to-day management of the staff as well as hiring, firing, and staff development. Her writings have been published in the Washington Post, the New York Times, Maxim, and dozens of other newspapers. She blogs at Ask a Manager.