Seth writes about feedback. Excellent post. While he's talking about all proposals, I think the really salient point, when it comes to creative, is this first rule:
The first rule of great feedback is this: No one cares about your opinion.
I don't want to know how you feel, nor do I care if you would buy it, recommend it, or use it. You are not my market. You are not my focus group.
What I want instead of your opinion is your analysis. It does me no good to hear you say, "I'd never pick that box up." You can add a great deal of value, though, if you say, "The last three products that succeeded were priced under $30. Is there a reason you want to price this at $31?" Or, "We analyzed this market last year, and we don't believe there's enough room for us to compete. Take a look at this spreadsheet." Or even, "That font seems hard to read. Is there a way to do a quick test to see if a different font works better for our audience?"
Analysis is a lot harder than opinion because everyone is entitled to his or her own taste (regardless of how skewed it might be). A faulty analysis, however, is easy to dismantle. But even though it's scary to contribute your analysis to a colleague's proposal, it's still absolutely necessary.
Young creatives stretch the limits of what's been done a zillion times when their older, more jaded creative directors (me), get out of the way, and let them think in ways we won't, or can't. Of course, getting out of the way doesn't mean anarchy. Nor is experience or creative opinion useless. But understanding (and applying) the difference between creative opinion and creative strategy is probably the most important element of decent creative direction.