From the article:
With Whisper Systems’ TextSecure V2, encrypted messaging just got a whole lot slicker, faster, and easier to use. V2 drops both SMS and MMS in favor of a fully private and asynchronous messaging system that sends texts over a data network instead of via SMS protocol. TextSecure now behaves a bit like iMessage, but with added features like unlimited group chat and media attachments.
As Marlinspike said, the team designed V2 so that it was as “low friction” or easy to use as iMessage or WhatsApp. Users simply want to download an app, set it up, and quickly being messaging, which isn’t how end-to-end encryption apps are typically designed.