You start to hear the generic code words like "not a fit" when there is something more to it, like we just didn't like you, or something perhaps bordering on illegal.
I have clients that deliver me very personalized rejections for some candidates and very generic rejections for others, at which point it becomes fairly obvious that personality and other things that a candidate would be sensitive to are often the issue. It's much easier to hear "your DB skills weren't up to snuff, bone up on those and you'll get somewhere" than "we didn't like you, change".
Fear of offending candidates, or being sued, are the root of the problem - and 'not a fit' is such a bland response that it generally just creates more questions from candidates.
What do you tell them that is constructive and not legally risky?
I am actually going to write a blog post about this during the weekend - will pingback!
EDIT: "But somebody who spent a day interviewing with you deserves more than a form letter."
This. Exactly this.
I disagree that said feedback is provided during a rejection letter.
I suggest to you as a candidate, to ask for feedback when they ask you if you have any questions for them... At that point, they can say you did well in this area, not so hot in another, etc. But its up to you to ask.
[1] Yes, you could say that I enjoy finding interesting-looking companies and send them my CV even if they are not actively looking for someone.
Only about 20% of people actually want to hear feedback. One of the challenges of giving feedback is that you want the decision to be final, but if you give points to improve upon then they are inclined to start negotiating with you.
With asynchronous interviews it gave me a very clear record of how they responded, which made it easy for the candidate to also look back at their own input.
One sales manager we interviewed had a significant background, previously having worked at high level positions with large companies.
However, when I saw his interview he was incoherent in his story, made vague statements, made several statements diminishing the challenges of the job without backing those statements up, and most of all his style of presenting himself showed he would be overly aggressive for the intended audience.
I gave him the feedback and instructed him to look at his own video with this in mind, and he really appreciated it.
The thing is, people need detailed feedback to improve. To stay safe on the legal side it would be easy to just give binary responses, but if we're really looking at what would improve the world, it would be actually detailed responses that enable people to improve.
I, however, reply with a more personal tone.