I think if there is a "hot link" to the page for filing a complaint about violation of terms of service, that would be a good thing. I don't think a button that "fast tracks" complaints or even one that "flags up" concerns would be a good thing, or workable.
It would increase "traffic" to twitter management, who seem not to have the capacity to deal with concerns already being raised. In the days before Twitter and Facebook, I was active in a small friendship/discussion board that introduced an "inappropriate" button, thinking this would help with moderation of "problem posts." The idea was that if a post got a certain threshold of "inappropriate" clicks, it would be temporarily deleted and the member could be temporarily suspended until the moderators could look into the situation. You can imagine what happened next and no, it didn't work. It created more work in the end for moderators and alot of ill feeling and accusations amongst the membership about who did what to whom. That was on a site with fewer than 1000 members.
Tunip I think it was mentioned that some radical feminists did not support the idea of a report button as they feared it could be used by those who disagree with them in an effort to push them off Twitter. It could be used by Twitter members who are popular or in the public eye to encourage their followers to "push the button" on contributions they don't like (whether or not they break terms of service.) In any case, even if the button is just to "flag" posts that could be dodgy, it will still be those who are already most privileged, most supported, most influential who will have enough supporters to generate enough interest in investigating a tweet or member. It will make little difference for the "small fry" of Twitter.
For those saying, "All manner of folks get abuse on Twitter," that is true, but in this case, we are specifically talking about people who are abused because of who they are and the nature of that abuse relates specifically to who they are - i.e. raping a woman, having a disabled person put down, Black people should be assumed to be criminals until proven otherwise, etc. It's not just having a go at someone for what they say. It's having a go because of who they are, and the threats are "backed up" by the views of many within society - not just some random rantings.