ignore what someone has to say just because it's negative is a sure way to ensure a major screw up.

The ability to spot and pick holes in plans is a great ability to have on any team as it means you don't end up with a project that's 95% done before you find out a basic flaw in it. Think about the Mars lander that got wiped out because no one thought to ask if the two teams were working off the same set of measurement systems.

History is littered with major screw ups due to small items that were missed at the start, so is the corporate world. Think about what Napoleon lost at Waterloo because he wouldn't listen to his artillery commander who told him the mud made using the artillery a problem and wanted to wait until the fields dried out before they started the battle. The lack of the often used artillery mobility was a major factor in loosing that battle and the war. The Battle for Arnhem would have been very different if people had listened to a couple of people who questioned the suitability of certain equipment and decisions; they were told to get in line with the program and not to be so negative - and thousands died because they weren't allowed to be negative about the project problems.

So think on those before you write off your Negative Nancies and Downer Dans, listen to what they have to say and give serious consideration to the problems they raise. Sure if all they say is "I don't like it," it won't have much weight, but ask them why and look at the details they raise.