The folks at Twitter feel that 140 characters in most languages is not enough to convey thoughts, so they doubled the amount to 280.
I don't use Twitter for conversations because of the limitation. I wonder if by adding more space, people might be conduct more conversations on Twitter?
Giving you more characters to express yourself
We want every person around the world to easily express themselves on Twitter, so we're doing something new: we're going to try out a longer limit, 280 characters, in languages impacted by cramming (which is all except Japanese, Chinese, and Korean).