I asked a question on sleep which I realized after it was brought to my attention in chat was overly broad. It's been deleted now (as it should have been) so I can't link to it, but the general question was "How does sleep work? Why is it necessary?"
It showed a few classic symptoms of a question that is too broad:
The question had multiple, loosely related questions
Namely, I was asking both "How does sleep work?" and "Is it necessary?". That's too broad. If you're asking one or two specific, related questions, that's okay. But two different questions about the same topic is a problem.
The question didn't have a clear, concise, correct answer.
"How does sleep work?" doesn't have a single correct answer, because it's too broad. What part of sleep? is part of the problem. Also, what does "how does it work" mean? It could have been made more specific by targeting what I was asking, for example, How are memories improved / strengthened during sleep?
The question could not be easily answered in a 1 - 10 paragraphs
Not that a single paragraph is ideal or that anything over ten paragraphs is too long, this isn't a hard and fast rule, but basically anything that requires an entire thesis or a book as an answer is too broad.
Now, this is my experience from other Stack Exchange sites and what is too broad 8on this site* is really up to us as a community to decide. We'll decide this mostly by example. As we find questions which we feel are too broad, we need to start a discussion about them. Chat is one great place to do that.
The actual questions we close as too broad during our early days will go a long way towards deciding what too broad actually means on this site.