It's the first step toward tying together a collection of desktop, Web and mobile apps that were assembled through a hodgepodge of acquisitions.
"It's a conceptual redesign of the whole product," Costolo told reporters at the launch event.
The company is laser-focused on two things right now, insiders say: Refining its product and making money.
This really identifies the heart of the problems that so many long-time Twitter users have with the Twitter website and iPhone app redesign: the focus is on Twitter as a product, and on making money. It's no longer on the user experience or on clear, concise communication between Twitter users.
The argument that "you're not [Google's/Facebook's] customer, you're the product" is very clearly applying to Twitter, too.