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I think the best solution is to have specific canned close reasons, with vetted messages clearly communicating what was bad, why that is bad, and specific tips for improving it.

Here are some attempts how this could look:

> This post contains multiple unrelated questions. 
>
> [explanation why that is a problem]
>
> Please split it into separate questions.

Or 

> There are many potential answers to this question, but we can't tell which ones apply in your case.
> 
> Please provide additional information as requested below.

Or 

> Doing justice to this question would require very long answers.
> 
> Unfortunately, few of our contributors have the time to write such long answers.
>
> Could you make your question more specific? 

The above suggestions are just first drafts, I am sure they could be improved further.

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I am also not sure that closing is the ideal process here. Our aim should be to give feedback, and reduce the visibility of poorly received questions, but not every feedback necessarily justifies preventing answers. 

A lot of close reasons are matters of judgment. Just how long is too long? Just how broad is too broad? People have different thresholds, and rounding that gradual scale into a boolean presumes a degree of objectivity that can not exist. (Case in point: my [second-most popular answer on stackoverflow](https://stackoverflow.com/questions/8481301/covariance-invariance-and-contravariance-explained-in-plain-english/8482091) was on a question that received informal "too long" feedback)

Put differently, I think there can be value in communicating why I won't answer while still allowing other people to answer.