I'd say, if the case is at least borderline-to-appropriate, then just make a comment suggesting to split the question, and otherwise go ahead and answer. If the case is really inappropriate, then -- in addition to the comment -- vote to close as "too broad", and possibly also vote down.

Though as this post is tagged as feature request -- the implementation of which feature do you propose?

You can flag for moderator attention, and leave a corresponding comment.

@FedericoPoloni: Actually both.

@CarloBeenakker: Does this clause apply to MathOverflow as well?

Isn't the main question here how many mathematicians asked a question on MathOverflow some time, and did not become regular users due to the feedback they got? -- And if so, how would you get useful information on this by asking on this site?

Well -- you can also vote on answers to closed questions. So I guess this is intended behavior of the software.