From Inside Higher Ed, former President Pierce has a piece on the problems and pitfalls of academic searches (like for university presidents); while some of it is academia-specific, much applies more widely to job searches:
In contrast to letters that are tailored to the position, nothing alienates a committee more than receiving what I think of as a “generic” or “to whom it may concern” letter, i.e. an all-purpose letter that candidates have prepared to send in response to every ad that strikes their fancy (and some that do not). Those of us who serve as search consultants know of a number of perennial candidates whose stock letters arrive usually within a day or two of the ad’s appearance. These letters tend to follow the same pattern. They begin and end with a paragraph that mentions the hiring institution’s name, but in the body of the letter they focus exclusively on their self-perceived strengths and accomplishments without regard to the college or university. Such letters typically end with a reiteration of how much the candidate wants the position.
My own favorite example of the dangers of such an approach came from a candidate who submitted her template by mistake. Her letter of application therefore began:
Dear [name of chair of search committee],
This is to apply for [name of position] at [name of school]. I am sure that I would be a great [name of position] at [name of school]. Indeed, I have spent my life preparing to be [name of position] at [name of school].
This application provided the committee with some welcome comic relief, but its members had no interest in the candidate.
Read the whole thing here.