I admin a forum with a little over 30k members.
In the past, we've had a former member we've banned file a false DMCA claim against us and successfully take us down.
We moved DNS and have been up since; For the record, 1and1 didn't even investigate, whereas our replacement, GoDaddy, kept a note of the circumstances and have notified us when that same person has tried to take us down again.
If this passes, our little forum is fucked. No two ways about it; Somebody will get butthurt about being banned and we'll get taken down, again - but this time, there's a risk of actual criminality behind it.
Don't tell me that this will be carefully used and no false claims will succeed, because we've been on the wrong end of that tale before.
Today, we're participating in the scheme being run by
http://americancensorship.org/; If you run a site, regardless of mission, you should too.