I think there are a few rare cases where keeping your idea close to the vest makes sense, but overall I think it's delusional to think your ideas are worth protecting.
Think of it this way: most, if not all ideas have already been thought of by somebody else. Are you damaged by those people having thought of the idea first, or are you damaged by them having executed it first? If they don't execute, then it's just an idea, and therefore it's of no value and of little concern to you. If they do execute, then they have something of value and it's something you should be concerned about.
The thing about ideas is that they are usually very one-dimensional. People will think of "foursquare for dating" like byoung2 mentioned but rarely think past that initial idea to things like what the competitors are, what are the barriers to entry, what will attract users, what type of design will work best, which framework should it be built on, has it been tried before and if so why did it fail, etc.
Of course, once you've answered all those questions you now have more than just an "idea." You have a plan of attack, a strategy, market research, design concepts, historical data, etc. In other words, you actually have something worth protecting.
As far as the bias on Hacker News is concerned, I think a lot of that stems from a couple of things:
First, there are two types of people in the startup world: ideas people and builders (not mutually exclusive, mind you). I think builders get frustrated by ideas people when equal value is put on the thinking up of the idea as the actual building of the idea. I can personally attest to this; I am constantly running into people who have a "great idea" but just need someone to build it (for equity, of course). It's hard not to get angry at those types of propositions. Every now and then I'll run across someone who has genuinely put in a lot of work on the non development/design side of things and has really thought about the ins and outs of an idea, contacted the right people, formed the right partnerships, etc, but it's rare.
Second, everyone on hacker news has ideas. Hundreds of them. I myself come up with at least one good idea a day. Also, a lot of us have acted on quite a few ideas and learned how much more difficult it is to be successful than one might initially assume. We also learn that lot of the mistakes we make with these ideas as we build them out could have been avoided had we subjected the idea to public scrutiny earlier in the development cycle. So when we run across people who are protective of their ideas it makes us scratch our heads - they're missing out on a critical component of becoming successful.
I could probably come up with hundreds of examples of sites that weren't first to market but now dominate, the most obvious being Facebook and Google. If ever there was a better argument for execution over ideas, I haven't seen it.
One final point: most people who have the capability to build good apps would rather work on their own idea, not yours.