This came up in the comments on Controlling magic through words - possible?, in which the author of the question did not want to detail background elements for fear that others might copy the idea. Based on that, a discussion insured as to whether or not utilizing the ideas presented in a question (or, in a broader context, answers) would make the work using the material derivative.
While not a lawyer, I do have a very strong (unflattering) opinion of copyright law, which has led me to take an interest in it. My understanding is that to constitute a derivative work, direct references would need to be made: either content would need carried over (characters, setting, etc.) or actual portions of the text would need copied (including translations). Indeed, only the second is entirely clear-cut, with situations like LOTR’s elves and dwarves being prominent staples of fantasy. Merely taking an idea and using it without direct use or reference to the source would not constitute derivation.
The only case in which (again, by my understanding) the Worldbuilding license would apply is direct reproduction of material: say, a Q&A book on common writing problems.
While this is perhaps a broad question for meta, I think it is important: if members here believe they can (and possibly intend to) take legal action if their answers are utilized, that needs to be made very clear to all involved, regardless of what the answer to this question is. I very much doubt the majority of people asking questions here intend to place their works under by-sa.
I think someone with direct legal background or perhaps a member of the StackExchange staff would give the most valuable response to this question.
Note that this is not the same question as Using content from answers?, which seems to refer to direct copying of material (as with my Q&A book example above).