i.e. the kind of people who find every reason why something won't work.
when you look at their accomplishments, it's always... nothing.
That said, negative nancies over someone who takes my idea and replies back passionately with the apparent problems with the idea and how to fix it--that's cofounder material.
startup lore is riddle with ideas that were ridiculed by people who couldn't see the vision early enough. (hotmail, for one)
If you wait too long and someone is working on a similar idea, you might lose early customers.