The other thing I think has made twitter more popular than it normally would have been is that it has been based in the positive. I can mark stuff as favorite, and I can follow people. I can't down-vote some tweet into oblivion. Harassing people is a little harder (you can block a person, but it is not listed publicly and it only affects your vision). Public categorization was not in the original design.
Now, we have lists. Most people are using them to make reading easier, but some are making lists with "less than kind" names and assigning people they don't like to them. I do wonder if that would have changed things if introduced earlier.