Saying what works as part of your criticism is also a pragmatic way to make the recipient receptive to your negative criticism. A friend of mine who is a professor uses what he calls the sandwich theory of essay criticism: Say something nice. Say the critical stuff. Say something nice. The tough and bitter meat is in the middle, and the bread makes it palatable. I’ve just taken over a peer-review essay site and added a feature (expert reviews) that has come to dominate what the site does. Although the reviews we have started offering are not scathing, they are detailed and very critical. My hope, of course, is that they are very useful, but some students who submit essays for review will find their writing dissected and examined to an extent they have never experienced before. And it’s in public. I expect that it may for some be an overwhelming or even harrowing experience, and reading this post and the comments makes me appreciative of the courage of those who submit their essays for such criticism.
Yes, the shit sandwich, as one trainer I had called it. If you have someone who uses the sandwich, each time he says something nice you start bracing yourself for the shit that follows. Personally, I believe you should be up-front about criticism, but be aware of how it will affect the other party. There's lots and lots written about this, so I won't trudge through it again, but keeping non-confrontational language focussing on the thing that is wrong rather than "you did it wrong" is usually better.
Like the previous commenter said, it is difficult, but the shit sandwich is not always the way to go, especially not if giving feedback to me.
Actually, Dr. Chris Nass (author of The Man Who Lied to his Laptop [1]) claims that people's memory of what is said after criticism is much better than what is said before criticism, so you should start with small praise, then criticism, then end with a larger amount of specific praise. He talks about it a bit in a Science Friday interview [2] and apparently more in the book.
