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I quite like this idea and have only once seen it implemented in a way that I think works well in a forum. The OP has the option to mark the thread as a question on creation, then the OP can mark as many replies as they want as 'helpful' and mark one reply as 'solution'. The top of the thread then has a drop down box where you can filter the replies: "All posts", "Helpful answers" or "Solution only".