User Posts can be downvoted but you need over 500 karma to have access to the downvote button. A Submission can not be downvoted.
Submissions can be flagged by anyone and mods/admins can downweight them. (If I’m not mistaken this is common for, say, Flock posts at the moment.)
Curiosity & repetition are two key factors.
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User Posts can be downvoted but you need over 500 karma to have access to the downvote button. A Submission can not be downvoted.