"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."