You can't steal what nobody owns in the first place. The law does not acknowledge that ideas for stories can belong to anybody... which is just as well, because if it did, the heirs of the authors of Gilgamesh would still be in court with the heirs of the authors of Genesis. So if someone wants to use my idea(s) for a story of their own, good luck to them. That just validates my belief that the idea was a good one.
Most of the "would you read this?" or "what do you think of this story idea?" questions I see on here aren't worth the bother of stealing. Most are clichéd and/or vague. Those that give the whole story often don't inspire me - which doesn't mean that there's anything wrong with them, just that I can't get excited about writing that sort of story.
To answer your specific questions -
Would I ever consider stealing an idea? A character's name is not an idea. (I really don't understand the obsession that some people round here have with finding the right name for a character. Just call them Fred and Ginger and get on with writing the frakking story. When you think of a good name, use find and replace to drop it into the document.)
I have a file of ideas that might one day find their way into a story, and I sometimes note interesting ideas from stories I've read. For example, one (otherwise unreadably dull) book had a prologue from the point of view of the evil overlord, who was looking at his calendar and thinking it was about time for another Chosen One to arise and try to slay him. He wondered if there would be a prophecy this time. He liked prophecies, because the Chosen Ones always followed them... so the evil overlord knew what the Chosen One was going to do, and so could easily defeat him. I haven't used that in a story yet, but I might one day. That's about as far as I'd go in using "someone else's" ideas. When I was plotting an earlier book, I noticed that some of the characters were uncomfortably close to characters in a series I admired, and tried to move them further away.
There was one question I saw here that I thought might make an interesting story, so I took a copy of it, but haven't done anything with it yet. It was about vampires, so I'd have to twist it and adapt it to make it my own.
Is stealing from an answer posted more guiltless than stealing from a question asked? - Hmm. A person who answers is volunteering information, so perhaps it would be less wrong (assuming you think it's wrong at all). But I think stealing is stealing and copying is copying, regardless of a person's reasons for offering the information.
Why would you NEVER do it? - That assumes I wouldn't. See above.
What genre do you write? - Fantasy and science fiction.
Do you steal ideas from anywhere else? - See above. Anything and anywhere can be a source of ideas if you can learn how to look at it properly. I got an idea for a story when I went to the shops one lunchtime and saw a group of people hanging around outside an office.
Is borrowing a certain grouping of beautiful words okay? - Depends on how expensive their lawyers are ;-) If people are likely to recognise the source, either acknowledge the source, or don't use it.
If you have stolen an idea, was it worth it? Did you feel guilt? - I don't like that word "stolen", because it implies that ideas belong to somebody, which they don't. I've used ideas from all over, but have done my best to mix and match and make them seem original. My earlier work was very derivative, but that's what you do when you're learning how to write. The stuff that I've posted for public consumption and am trying to get paid for... I don't feel the slightest bit of guilt about the non-original parts of it.
Have you posted your storyline on Y!A to be approved? - What, like I need you guys' permission to write?! I did post a pitch for my current novel here (not the whole plot, more like the blurb on the back of the book). Most people here loved it. Then I showed it to my critique group, who tore it to pieces. Make of that what you will!
EDIT: Liam, I hate to disagree with you, but if you're right, you'd better tell Tor Books that they're wasting their time and money printing copies of Cory Doctorow's novels, because he's busy giving them away for free on the Internet.