There are a lot of unscrupulous developers out there who will clone a game to try to cash in on its success: see for example, all the different versions of 'threes' or 'flappy bird' on the app store.
Notice however that I said they will clone a game, as opposed to an idea.
These people are in the business of trying to make a quick buck, and they want to minimise the risk of wasting their time and resources. In order to minimise their risk, they typically copy successful games. They aren't interested in an idea that may or may not actually be successful.
There are also some people who will see an unproven idea and copy it if they think it's good. It's probably rarer than you think, but it does happen. A lot of these people are beginning developers, and you don't need to be concerned with them, because most will either not finish the project, or will release a typical low quality beginner project that won't compete with a better version.
Consider also, that if you give the exact same idea to five different developers, you will very likely receive five completely different games. Just look at the different interpretations on the themes for popular game jams!
Lastly, even your most original idea has probably been thought of by some other people. Surely you've had the experience where some developer releases a game that's exactly or very similar to an idea you haven't told anyone about. If an idea is even half decent, other people will think of it without stealing from you.
In summary, I don't think having your idea stolen is something you should worry about. It is my opinion that the value you get in feedback, potential help, generated interest, etc. far outweigh any risk of idea theft.
Successful indie developer Daniel Cook shares his thoughts in this very well written blog post: Why You Should Share Your Game Designs.
HERE is a recent discussion on the topic in our forums.
Now, all the above being said, withholding some information for marketing reasons can be valuable. You almost certainly don't want to keep the whole project secret until release, but you might have a marketing plan that involves withholding certain features or aspects of your game until some formal announcement. This generally works better for established developers with a large eager fan base than someone unknown, but it may still be something you'd like to try.
If you're withholding information, do it for the right reasons, not because of some fear of having your idea stolen.
Hope that helps!