Well, yes, but. You also need an idea that’s worth bothering with. Paul Graham points out (at some length) that you need to go for something painful that other people think is too hard or difficult and then find a novel and easy way of doing it.
There are millions of “me too” “Web 2.0” “social networking” sites that do nothing and eat VCap cash that could have been spent on something that sucked less.
So yes, ignore the nay sayers, good! But make sure you’re doing something original enough to make money too (even if original means cheaper and better).