Street Fighter has traditionally been a 2D fighter. Capcom has traditionally been a 2D fighting game company. That is, until Street Fighter IV.
In a break from the previous Capcom developed titles, Street Fighter IV introduced the venerable series to 2.5D. It's somewhere in the middle — not exactly 2D and not exactly 3D. "The 3D models on a 2D plane is a very good match and all of the (game) systems are capable of supporting such a feature," Capcom producer Ryota Niitsuma told Kotaku in a recent interview. Niitsuma is heading up Tatsunoko vs. Capcom: Ultimate All-Stars, a title that like Street Fighter IV uses 2.5D.