To follow up on the idea using content censored elsewhere as a marketing pitch as a new comment thread.
I completely agree with @nxtblg that simply opening up the flood gates by saying 'Bring your censored content here' would more than likely cause issues, whether copyright concerns, hate speech, etc. There would be little benefit to causing DMCA problems for steemit as well as drawing in people who then just would find themselves flagged on here as well.
To keep things reeled in a bit, I think we can pick a few select topics with the 'disclaimer' of what will get you flagged (such as ad hominem attacks) with the goal of not only being a draw into Steemit (for the conversation) but also being able to 'train' some of these new users how to actually argue their points of view in a civil manner. To some degree we already saw this occur here on steemit on 'feminism and related power' from @veralynn. While there were things that were flag worthy, there was also great discussion with personal, logical backing.
Some topic ideas for this:
Non-Americans view of America (there are valid perceptions and arguments available without degenerating into hateful/violent speech, and honestly something I think is beneficial for both sides.)
Flat Earth theory (such as this video on bittube)
While not necessarily censored, we can also try to strike up the discussion on things like zero point energy or other more fringe theories.
Thoughts? or better topic ideas? Imo they have to be something that invokes people to state their point of view, but still have to do so in a rather civil manner.
Thanks for your fantastic response. Give me some time to think on it. I do really like the idea though.