I keep wondering this: if somebody with 13 rep writes an answer instead of a comment (because they can't) is it appropriate to downvote that?
My answer in short - it depends entirely on what they've posted.
I remember starting on here and being very frustrated that I couldn't comment (I'd been a reader of SO for some time, so had a pretty good understanding of how things worked before I started). Okay, 50 rep isn't a lot, but it's still a fair barrier against a brand new user interacting outside of the first few questions/answer that they post.
It's a balance we need to achieve between preventing clutter/noise on the site and making sure we don't discourage new users. And it's a difficult balance to get right, particularly considering so many users can vote down, and most of them will not be involved in this conversation.
Personally, I'm more inclined to go with a gentle reminder, where suitable, but ultimately it depends entirely on what they have posted:
- If it adds value then it should be kept live and therefore doesn't deserve down-voting, but nudging the user to re-post as a comment (maybe at a later time) is the way forward to keep the site tidy.
- If their comment adds nothing then it deserves deletion, and the best way to encourage that is by a down-vote (unless it's also offensive/etc in which case flagging is better).
Remember that if these users delete the response that they will get all lost rep back again (possibly after a recalc?), as will those who downvoted. So this is not necessarily permanent damage to the users rep - unless, of course, we scare them off.