When critiquing a work in progress, it's important to heap on the praise, in addition to saying: 'I like this a lot; I like this less; this is confusing; good dialogue.' Because, while the writer will be grateful for your suggestions and honesty, most crucial is the unasked question: Is this a crock of baloney? To which the writer will usually reply ' yes' about his or her own work.
Then the writer will think: So and so gave me great suggestions but I've barely heard from him/her since. They must think my writing stinks, my play (novel, story, etc.) is so bad they've decided they can't bear to remain friends. They've seen who I really am: a fraud, a bad writer, a self-deluded, untalented, pathetic... And it's very hard to go back to the work without thinking: I haven't heard from so and so in a long time, in fact since they read my draft! Is it really so awful? Obviously, it is.
Whereas, critique plus praise (no matter how manufactured) helps the writer when s/he sits down to the unwieldy, awkward, horrible-seeming manuscript.
Praise is essential in all endeavors, unless one already possesses an incredibly resilient sense of confidence -- and these folks usually sing their own praises, and they usually aren't writers.
