Content-wise, I don't think any service is immune from that - Reddit can be pretty horrible and I was shocked by some of the stuff posted to HN Tuesday & yesterday (though it was quickly cleaned up).
My problem with Twitter is that the brevity and emphasis on the present moment makes for bad social interactions; the faux-urgency of social media in general elevates sensation above substance. It's so easy to get the Internet Hate Machine going, and offers such large ego rewards for people who feel they are able to shape and direct it, that it's become a regular part of discourse. Back in pre-web days when NNTP was in regular use, discussions of contentious subjects were just as vigorous but nowhere near as vicious or reductionist.
Our ability to project our own ideas has significantly outpaced our willingness to consider those of others, it seems.