When Ideas are down-voted, users should be forced to make a comment explaining why. An up vote clearly means that they support an Idea, however a down-vote doesn't give any indication as to why.
Justification for this Idea:
Lets take an example scenario - your boss calls you over at the end of a project and says "Good job, you did good!", you're not going to ask why, are you, its clear that you performed well!
Alternatively, your boss tells you "You're sacked..." but doesn't tell you why - wouldn't you want an explanation, to know where you went wrong?