Josh Wilburne, Designer at Twitter:
With this in mind, we designed a system that defines two types of written languages, dense and non-dense, and expands the character limit for non-dense languages. By grouping languages this way, we can give people writing in non-dense languages like English and Spanish the same space to express themselves as people writing in information-dense languages like Japanese. This will make sharing thoughts and ideas on Twitter a lot less frustrating for many more people, while maintaining brevity on Twitter overall.
Twitter’s recent character count increase isn’t as simple as 140+140 (even though Jack tweeted thus). They seem to have done this thoughtfully. I think once the #280characters meme dies down a little things will settle in nicely for Twitter. I’ve already seen some great cases where it was used.
I also very much like the UI they’ve designed for showing you your progress which Wilburne covers in his post.