For one, the lack of any nuance. 240 characters is not enough to accurately articulate a persons thoughts, so people infer meaning from otherwise harmless statements. Places that allow for longer comments, like HackerNews, have better discussion because you can post what you actually think.

Second, I just don't like the site. You go to the site, click on the news ticker for any topic, and you see a million of the worst takes ever dreamed up by mankind. The format also encourages mob mentality which I find very disturbing.