There are questions which are badly written and absolutely unclear. Those are downvoted and closed as unclear within minutes.
There are questions which are clear but I personally can't understand them because I lack some knowledge. For instance, I don't understand most questions about Haskell because I don't know Haskell yet. Obviously, even if I can't understand those questions, there is no reason to downvote/close them.
Finally, I remember a few cases where a question is well formatted and looks like being clear, but after reading it several times, I still can't figure out what is it talking about.
A practical example is the recent question Code structure with MongoDB in python. I've read it four times. I know MongoDB and Python. The question looks right. Still, I have absolutely no idea what the person is talking about.
What should be the behavior in this case?
Be subjective and downvote the question and vote to close?
In doubt, do nothing and let other people deal with it?
Request clarification from the author? In which case, what should be the comment? I doubt that a comment which tells that the question is unclear is constructive.
Close the question as unclear (given that four other persons should also vote for it to be closed), but avoid downvoting a question which may be perfectly valid?