Google "how to write a great query letter" and you'll find hundreds of articles chock-full of advice for the aspiring author---"cut to the chase," "sell your story," "don't be arrogant," etc.
On the flip side, there's a dearth of advice for agents looking to write a rejection e-mail or letter that strikes the perfect balance between crushing an author's self-esteem and providing the slightest shred of hope for future success.
Kill them softly
There's no need to tell a querying author that his work "just isn't interesting enough" for you. It rings equally hollow to tell him that you "didn't feel passionately enough about the work" to request additional pages. How about something like, "it's not among the genres I'm currently representing," or "it's not a genre in which I am currently seeking new authors"?
If you feel the need to discuss your lack of passion, reframe the message with a clever or better yet obscure reference, such as, “I found your pages warm and fuzzy, but they didn’t put me in the tall cotton,” or “While you certainly provided me with a lot of hay, it wasn’t all in the barn.”*
Use your adjectives
Do the exact opposite of what newbie authors are told to do, and use adjectives to soften the blow. Instead of saying, "I'm afraid I didn't connect with the material" (a common mistake among agents), why not say something like, "While I found the material ____________ [insert adjective here, such as fascinating, interesting, thought-provoking, descriptive, etc.], I didn’t quite cotton to it.”
Say something positive
Most agents conclude their rejection e-mails and letters with something like, "Our business is subjective by nature, and another agent may well feel differently.” In other words, “It’s not you, it’s me, but it’s really you.” Why not conclude with something more inspiring, such as “Never give up, never surrender,” or “May the Force be with you?”
*for information on these types of euphemisms, see my prior blog post about how to write like a Texan.