Great discussion. I like this idea in theory. One of the pitfalls I see, though, is that if someone developed a vendetta against someone, they could then downvote all their material, even if it's high quality. To that end, I'd discourage liberal downvoting and maybe set a limit between 5-10 downvotes per day and/or incorporating some sort of algorithm that would search for and prohibit abuses of the downvote button.
I contribute regularly at Quora, where there is a downvote button, and I think a similar model could possibly work at Steemit. I have downvoted comments on my own posts, for instance, that were outright nasty. I had an answer that went viral last year that invited many positive comments, but it also invited haters. Downvoting collapses the comments, which a user has to manually expand in order to view them.
Similar to Steemit, you can also flag content on Quora, so it would be important to differentiate when a user should downvote versus flag. My suggestion would be to use downvotes to collapse negative / unhelpful comments and use flags for posts for plagiarism and low quality. And that model could work on Steemit: you could continue to flag posts but you would not be able to downvote a post. You could, however, downvote a comment. The negative curation rewards would then kick in on comments only. I think it's important to make distinctions as some people make their living on Steemit, so you have to be careful when it comes to people's livelihoods. A great idea, though, and I wouldn't be surprised if Steemit tests this at some point.