I started to write a comment on another answer but realized my thoughts might be worthy of their own answer. Basically, I think @BoogaRoo is more or less correct that downvotes often happen because people think "golly this is a bad idea."
Personally, I try to take the approach of seeing things through the asker's perspective, and assuming they have the best intentions. Or at least, innocent intentions. In other words, to them it may not be a "bad idea" they're suggesting or asking about.
This lets you separate evaluation of the question (it may be a "good" question by TWP standards) from the idea (it may be a "bad" idea). Then, I address bad questions by editing, commenting, and voting. Address bad ideas by answering to point out the badness.
We have to keep in mind that there are people from all backgrounds, with all different perspectives and experiences, gathering on The Workplace. In my opinion, someone asking "should I X?" that causes us to think, DUHHH!!! Why would anyone ever X? deserves an answer just as much as someone asking a question we actually have to pause and think about ourselves.