I would like to use 99designs to design the logo for a new site I'm working on that is still in stealth mode. Are there any dangers in doing this? Specifically, I am worried about:
It is a concern if your logo exposes the purpose or general idea behind your website or if you're describing your purpose in great details to those who'd be designing your logo. I'd use minimal information as possible that would still be enough to "inspire" a graphics artist in the right direction.
One thing all founders are over-anxious about is the safety of their ideas. In reality, if you scream your idea out-loud in grand central it's not enough for someone to hear it - someone has to invest the work to execute to steal something from you.
Ideas are a dime a dozen. Execution is rare. I wouldn't worry if I were you. This site works well for a reason. The graphics artists there are not lurking to get the next idea from a logo design gig. They just want to supplement their income by doing what they do best - graphics.
While I do agree with Ron in his answer, I will be open in saying that I know someone who goes through these kinds of sites looking for business ideas. While it's only one of so many, it's still enough if somebody has the money and the resources to take the idea and run with it.
The best advice I could give is that you make sure you have documentation of your idea that's dated. Then tread softly and as quietly as possible while in development so you don't give your idea to someone. That's probably your biggest concern. Don't worry. If you do due diligence on your end to ensure that your idea is documented, then if something is stolen you have something to use as recourse to re-coup your losses.
But in my own experience, most of the people in this world are happy to do a day's work and go home. They really don't want to steal anybody's ideas. They just want to get by. And that's fine for those people who have the desire to change the face of business in their industry.