On meta I believe there is a convention that one is voting for or against the sometimes implied suggestion of a question, rather than the quality of the question itself. What is the appropriate way to respond to what you consider a good question but a bad idea?
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Simple: you down vote the question, because as you mention that signifies I disagree on Meta. There is nothing forcing you to write an answer or leave a comment, but you can do so if you can explain succintly why you have up or down voted the question; people will then vote on you anser depending on its merits - you may have something in your answer that the OP never thought of which impacts on the viability of the OP's suggestion.