One problem I often encounter in the startup world is the tendency to push things to Manichean extremes. Perhaps its because we have to live with so much uncertainty, we crave definitiveness. Or perhaps its because the kind of mind that can write and debug thousands of lines of code likes binary answers.
At any rate, I often run into conversations where a person will advocate X. Then, when I point out a few minor issues, the immediate reaction is to abandon X. This makes no sense. Decisions are rarely clear cut, and allowing any negative feedback to shut down a course of action means you'll miss a lot of good options.
Or, as Obi-Wan Kenobi put it, "Only the Sith deal in absolutes [except for me, in this very statement, of course]."
Criticism is important in the startup world. We are trying to do things that haven't been done before, and we are likely to fail. 90% of the time, criticism is justified, because 90% of the time, we're likely to fail.
But when you criticize, you need to learn the distinction between being negative and being realistic. We've all met the guy who's against everything, and seems to take delight in tearing others down. Instead of saying, "You're wrong," which is both personal and unmeasured, it's best to be specific. "I'm not sure I agree with some of the assumptions that are implied by that plan." You can raise legitimate criticism without being negative.
By the same token, when you are criticized, you need to exercise the same restraint. It bugs me when the reaction to criticism is the petulant, "Okay, we just won't do that then." This is essentially the playground tactic of, "I'm going to take my ball and go home." It may feel good for an instant to perform this childish act, but it doesn't help your startup.