I have notices that sometimes a single unpopular question eats all user reputation, for users with less reputation, it may lower their reputation so that they can't even downvote or they loss other privilege that based on reputations.
I don't want to link any question here because I think its not appropriate, but I saw some examples in META.
My suggestion is that: allow users to downvote and other basic privilege when they lost reputation because of one question, because questions can't be deleted when had answers, and the principle can be based on previous reputation in some point.
USER A get 200 reputation, then he/she can downvote, after this point, if the reputation decreased below 125, allow him to downvote.
User A answered 10 questions and gained 100 point, but asked one question that was unpopular and it causes user to loss all reputations.
In this scenario, the user lost all reputations and now he might not able even to downvote, I see that this is unfair some how, but its also realistic because its all about how this website works.