Telling your boss why you are leaving.

[CosmicPossum]

Do not put any of the negative stuff into writing--it can come back to bite you!

After you have written up the first few reasons, and you feel that it might help, have a conversation with your boss.

[Gwywnnydd]

Shoo, while I agree with this line of reasoning in certain situations, I also think that exit interviews are a very important part of the termination process.  Good companies (or companies trying to improve themselves) want to know why their employees are leaving.  It's pretty standard.  Good companies can take the positive aspects along with the bad.  Any company that asks for an exit interview, I am inclined to believe either is a good company or wants to improve in some way. I have limited knowledge of the OP's situation, so perhas it is a really oppressive environment?  

While I agree that good companies want to hear honest answers in an exit interview, I have to disagree that the asking for one indicates a willingness to *do* anything about what is said. I may also be jaded, but I've seen too many companies that have exit interviews because it's policy, not because they intend to implement any changes based on the results. I wont even bother with them anymore, in most cases if the company cared to change this stuff they would have done so when the current employees were complaining about it, not waited until someone was leaving over it.

To the OP: only document positive stuff. My hackles are going up with the boss asking you for this stuff, it just seems like he's looking to put someone's behind in a sling, and it might be yours if he doesn't like what he hears.

[EvilAlice]

Maybe you could use a combination of the two different strategies suggested here.  You're taking the new job because of how well it suits your needs, but you were probably LOOKING because of issues where you are.  So maybe this:

"I was hired to do specific things, three different areas in total. A month after I was there, the job description changed completely.  This prompted me to search for something more suited to my talents and passions.  I enjoyed working with you all, but I have to take advantage of the opportunity being offered to me."

It's honest but not accusatory.  It can't come as a shock to them that changing job descriptions on people is not a good thing, and you won't look spiteful for pointing out something that had to be obvious.  This gives them an honest but very discreet answer, and you've given them what they requested.

[Dragons 8 Cactus]

Yes, Be honest.
They can't fix what they don't acknowledge .
dragons8

[ShadesOfGrey]

Maybe you could use a combination of the two different strategies suggested here.  You're taking the new job because of how well it suits your needs, but you were probably LOOKING because of issues where you are.  So maybe this:

"I was hired to do specific things, three different areas in total. A month after I was there, the job description changed completely.  This prompted me to search for something more suited to my talents and passions.  I enjoyed working with you all, but I have to take advantage of the opportunity being offered to me."

It's honest but not accusatory.  It can't come as a shock to them that changing job descriptions on people is not a good thing, and you won't look spiteful for pointing out something that had to be obvious.  This gives them an honest but very discreet answer, and you've given them what they requested.

I guess I have been pretty lucky to be associated with companies that actually care, and strive to provide a good working environment for their employees. It sounds like a lot of people think this is just a tactic/could come back to bite the OP. 

That being said, I think the quote above is a really good example of tactful but honest.