The original was released in 2010 and was received well by fans. People loved the idea of Sega's greatest characters coming together. With such a breadth to choose from, one could say that the possibilities were never ending.
Two years later we got the sequel, which literally transformed the racing genre with the different vehicular transformations to make them all-terrain. This was done the year before in Mario Kart 7, but that was merely the addition of gliders and propellers. The way courses changed over the duration of the course was also something I'd never seen before and this was a really cool precedent that I have yet to see implemented in other racing games that followed it. The roster also increased a fair bit, adding some characters outside of their own such as Disney's Ralph from Wreck-It Ralph, which is getting a sequel this year.