Before answering this question, consider the context in which Andrew Keen made the comment to 1938 Media's Loren Feldman: "I like Twitter because it's more civil than blogosphere".
Keen followed the line with: "Just block him". That's because Feldman was having a set-to with another Twitterer called Aronado in full boxing-match warm-up press-conference mode, the ones where after standing on the scales each calls the other a small cat. I can see Feldman's point after reading Aronado's bio, however. Zen of Kicking Ass? It's not quite as hopeless as the drudge who claimed to melt faces with his face-melting guitar riffs, but it's more or less on the same page.
See, already I'm off Twitter – where requests to borrow sugar are at least met with helpful advice on where the nearest supermarket is rather than "STFU noob" – and the bile is flowing.
So, back to the question. There are three possible answers to this:
a) It isn't. It's an illusion of the echo-chamber nature of Twitter
b) It is. It's in the nature of how Twitter works
c) It won't be. Usenet, forums, chat rooms and blogs were all neatly tended gardens once. Wait for the mob to arrive.
I can't make my mind up right now. I reckon all of the above are true to a point.
There are some arguments for Twitter being a more civil environment. As Charles Arthur points out, there is the Block. This is the facility that lets you stick your fingers in your ears and shout, "La la, I can't hear you," without all the effort. If someone gets enough blocks against them, their account gets killed. That's one reason for staying nice.
Second. This is a bit counter-intuitive but the 140-character limit may be a help. If you look at the way that forum discussion descend into abuse, it generally takes a while. And the length of posts gradually increases as the opponents take their turn in fisking each other before getting down to name-calling. To have a serious argument in Twitter, other than low common abuse, you have to take it outside.
Third. I reckon that for any network that demands you make some kind of formal connection, the participants will be on average more civil to each other. I'd be curious to know how Twitter conversations differ from those on Facebook. But I'd bet that Facebook groups descend into bitterness faster than personal pages, except where you have some concerted attempt at bullying.
Don't miss the first sentence in Keen's tweet - there's a lot of information in those 140 or so characters: "Sounds like the same problem Calacanis has. That's a shame". Jason Calacanis has more than 60 000 followers: there are bound to be some trolls in there somewhere. As these networks get bigger, the number of trolls and spammers approaches a critical mass and the tone heads downwards.
But, a lot of the time you don't see it if you have Twitter set up so that you don't see the @ flames. And, because people tend to cluster in interest groups, they don't regard comments against The Others as being insults or uncivil. The Skittles wheeze of putting its Twitter page up underneath some promotional blurb encouraged some to deliver the odd insult and as a young J Rotten once put it: "A rude word". Civil? But I'm not bothered about the feelings of carnauba-wax covered sweets, so it's OK by me.