Twitter’s Aliza Rosen, writing for the company blog:
In September, we launched a test that expanded the 140 character limit so every person around the world could express themselves easily in a Tweet. Our goal was to make this possible while ensuring we keep the speed and brevity that makes Twitter, Twitter. Looking at all the data, we’re excited to share we’ve achieved this goal and are rolling the change out to all languages where cramming was an issue.*
During the first few days of the test many people Tweeted the full 280 limit because it was new and novel, but soon after behavior normalized (more on this below). We saw when people needed to use more than 140 characters, they Tweeted more easily and more often. But importantly, people Tweeted below 140 most of the time and the brevity of Twitter remained.
I can see where more characters would be useful, but the timeline’s brevity is paramount. The (old?) 140-character limit is a core tenent of the service’s strengths.