Here's one: What if someone takes a poll asking if they should kill themselves?

I suspect people would react against people asking that regardless of whether they include radio buttons. If I recall there has even been drama surrounding making observations about a former member suiciding. I'd be somewhat surprised if someone asking this question directly did not prompt that comment to be banned.

The questions will be limited to whatever the poll creator types in, but that doesn't mean everyone will use common sense while creating their polls.

No, I haven't observed common sense to universally constrain posting behavior in general. However explicit polls don't strike me as sufficiently different or more powerful than regular comments, (inherently anonymous) votes and private messages that a move from informal expectations that people don't behave like @#%$s need be changed to a formal "Terms of Use".