Twitter has been making several transitions to their web interface recently, few of them have been changes to things people have always been familiar with. Quite sometime back they change the Catch line of Twitter from "What are you doing?" to "What’s happening?", and now a similar change has been rolled out where the "Update" button has been replaced with a "Tweet" button.
The change though subtle, shows a huge change in the mentality of how people see Twitter as. The "What’s Happening" change conferred upon users that Twitter was not just about what people are doing, and had changed to what is happening to them and around the world. The recent disasters in Haiti and Chile were good examples on how people were talking about "What’s Happening" around them.
The new "Tweet" button also signifies a change where people are not updating their status but are tweeting about different things. Definitely a good change. What do you think?