When I look at the First Questions queue I often encounter questions that make perfect sense, so the 'question is unclear' response isn't really appropriate. What the question needs isn't clarity but detail. An example from Blender SE today was a question that asked, paraphrased: "why does my mesh disappear." I end up marking such questions as unclear, then hunting them down in the questions queue and adding another comment asking for more detail.
I also find that new users don't know what to do with 'unclear' and get frustrated when they think the question is clear but haven't realized that they need to add detail before it can be answered.
For workflow purposes, I think it would be useful to have another option 'question needs more detail'; especially if it comes with the ability to specify what detail is missing.