This sounds like a great idea. Adding this to the FAQ doesn't explicitly discourage or eliminate any specific topics but is instead geared towards helping people take their real, actual problem and then focus on describing the problem in a very clear and organized manner.
This also makes the asker think about what he or she expects to receive as answers, which I hope would help reduce the extended discussion and amount of not constructive posts!
UPDATE:
Since this is now listed in the list of off-topic questions, it's important that we look at the question and determine if edits can bring it inline with the scope. I'd hate to see users give up, thinking their post is simply off-topic. Instead, I'd prefer they understand that some editing is needed to make it a great question. See this post for more detailsthis post for more details.