tbh I never get this "idea is precious" debate! I run a popular Mobile Apps related monthly event in London and time n again meet/talk to people who are very protective about their idea... I usually document my learnings at guides section of appsjunction.net. Here are my observations as an industry watcher/speaker and mentor/adviser to few London based App startups in this space..
1) An idea is more or less worth nothing, its all about execution, growth hacking, marketing strategy, hiring the right people, motivation in team, continuous innovation to stay ahead of competition, gap analyses, finding new markets, opportunity seize over and many such things which keep your graph going up or atleast horizontal on investors' scale).
2) if you dig deep into copyright and patent laws, you can't protect 99% of ideas anyway.
3) as soon as you launch it, anybody can copy it in max 2-3 months anyway (e.g. google groupon clones). for those 2-3 months, its not worth the hassle of not discussing it with right ppl/advisers/experts and get the right advice at right time as it costs a lot to re-invent the wheel..
4) NDA agreements are just hype. Ask any experienced lawyer in this domain. They are nearly impossible to implement. Also they are legal documents with different definition and implementation in different parts of world.
5) I recommend the best approach is to actually discuss you idea with not just industry experts but also with your target audience and find out the market-product fit early on before spending even a penny of hard earning money into that. Just like when you think about investing significant money in a car, you discuss your choice with the people you think can add value to your decisioning process... make an intelligent choice here,, :)
6) Am an app/web domain mentor. If your idea was about something else then my suggestions here may not be a good fit., Please seek advice of experts in your domain. You are always 10X better off with a mentor than not having one. Try someone in your region and area of expertise with good recommendations and if after listening to your business idea, if you get one good credible guide, that is like half battle won.
~ views here are my personal opinions, they don't represent the entities I am associated with ~