Not just for plagiarism and scams, but also for poor content, hateful/antagonizing content, or content that is receiving WAY more funds than what you would find appropriate.
In a recent example, a user posted an article that had a generally ok message, but there was a lot of implications and reading-between-the-lines that I found to be very unethical and callous with a message that potentially attacks emotionally scared individuals. Perhaps I am the only one who feels this way, but it is all of our duties to act on what we feel is right so I downvoted that post.
Do not succumb to pressure or bullying!
After I had downvoted this user's post, the user changed their post headline to attack and misrepresent my action of downvoting. You can try to bully or scare me into changing my vote, but it won't work. My vote is what I feel is right, and all you can do is take it for what it is, or ignore it. But trying to demonize / bully people into agreeing with your messages is not only very unappropriate, but will not work on user's like myself.
And I encourage everyone to not back down to people who confront you for downvoting etc. Do you what you feel is right without letting the author attack or condemn you into changing it!
This platform will survive on the ethics, morals and effort of us all. It is our responsibility to curate content, so don't be afraid to downvote!
Edit: 
made a very valid point about not downvoting EVERY post you don't like, but the ones that you think would devalue Steemit in one way or another:
I don't hesitate to downvote content that I think decreases Steem's value