Genre-Killer: This game's reception was pretty much the final nail in the coffin for fully 3D Sonic games. Starting with Sonic Colors, the series would instead adopt a style more like this game's daytime half.
Early concept art for Sonic's transformed state was a Yeti◊
Meta-example here; The Hedgehog Engine was originally intended to be licensed out to other developers. This never happened however, and it appears to have been completely internalised as of the release of Generations.