A number of new revelations have been mentioned, including that the game will also take players to China and Africa in addition to revealed stages Greece and "Europe." There will only be one other playable character which is used briefly for short sections (like mini-games) as the game is played for "99% Sonic in his two forms" (regular and Werehog).
The most striking details come from the day and night gameplay. Nighttime stages, the time when Sonic transforms into a Werehog, will appear in some stages as both real-time day-to-night changes and also fixed night or day stages. One last tasty morsel is info on a demo: "Yes, the current plan is to release a playable demo shortly before or right around release." There's a lot more info to read up on at the Joystiq interview so go read it up!