The thing that makes Twitter stand out (YMMV depending who you follow) is the 140 character limit. To use that effectively you need to be bright, literate, and be able craft a succinct message.
In the hands of intelligent people that encourages some brilliant writing, some sparkling wit, and frequent moments of insight.
There's a lovely moment of quiet pride when you craft a perfect 140 character tweet and see the retweets cascade.
280 characters? SAD!