Most of Silicon Valley (and all other sensible startup communnities) now really find NDA's to be taboo. I can't place where I saw it, but I'm sure there have been several blog posts by very notable VC's and angels that say if you make them sign an NDA they already know they won't fund you. As far as I know, this is somewhat of a new trend.
The principle behind this is that successfull startups are about execution, not ideas. And if you're so caught up in thinking the only reason why your startup is going to have an advantage is the idea, than you must be delusional and not really understand how startups work.
I have to say that I pretty much agree with this. And in addition, being open is not only not-hurtful-to-you but it can actually be helpful - to both you and the world.
Being open allows you to discuss your ideas with smart people, talking transparently about the potential opportunity and pitfalls. This will help you see problems that you wouldn't normally be able to see yourself, and possibly open you up to new ways of looking at things. And maybe your discussions will inspire someone else to look at your industry and think of solutions they can provide themselves. Either way, it will allow you (or someone else) to create something that will ultimately provide more value for the world than if you had kept it all to yourself.
The only thing is... even knowing this, I feel it is only natural to be hesitant in talking transparently with competitors or potential competitors. But the ironic thing is that they are probably the ones that can help you the most. They have done just as much, or more, research in the space and should know it as well, or better, than you do. So why not openly talk through problems with the people who are trying to solve the same problems? Here are just a few potential reasons.
We are naturally competitive creatures. I think instinctly, it doens't make sense to hand over ideas to competitors (big or small). We all want to win.
Sometimes it could be suicide. When dealing with bigger companies with more resources, if the idea you have really is that innovative, they could just either out-execute or out-last year. Either way, sometimes it's signing our own death certificate. Though most likely, if they're that big they're probably too slow and also your idea probably isn't that special.
So there are definitely reasons why someone would not want to be that open. But overall, being open and transparent will only help you more than it will hurt. Though sometimes I guess it can still be hard. And maybe I'm just being naiive.