Dear cainite,
i just came across this wonderful session about communication, maybe this is helpful:
http://www.llresearch.org/transcripts/is..._1205.aspx
i would also like a few words from my experience.
i have been in the Music Business for around 25 years and very often came across a situation like you described, being on both sides every once in a while.
When somebody asks for feedback and i dont like that piece of music, i usually am very careful in my answer.
i First take into account where somebody is at in terms of development.
To a 30 year old that has been composing for 15 years i would say much more directly that the Song is not really my taste, i would however also comment on "objective" matters, like if i think the Composition is strong or Even Point out weak areas that i perceive.
i would in any case try to honor the Love that was put into it, since every musician (or artist) will really Open itself up and show himself quite vulnerable.
If it is a younger or less experienced musician, i would greatly downgrade my expectations.
I would try to point out what i like about the song and how it is done.
i would carefully point out the weakest areas, but in a real loving way as much as possible.
i would try to give useful tips on how to improve and try to view more the technical side instead of being too person.
i have faced all Kinds of criticism regarding my music.
Especially in my beginnings, i have learned much from people that would honestly say whats not good, while also stating what they liked.
the feedback that was offered honestly and openly while still being encouraging was extremely helpful on my path and i am very grateful for that!
But still After so many years, when im being asked for Feedback and don't like it, it is always a big challenged :/
by the way, the typical "Friends feedback" (which was rarely honest) where everything was just great was in absolutely no way helpful.
i think musicians or artists in general are empathetic enough to tell the truth from a lie...