So there are 13 zones in the game, with 8 of them being remastered zones from previous game, and 5 being totally original zones. But there are enough changes in the remastered zones (like secret paths, enemies, music) that it feels like a whole new experience.
Yup, the mechanics are exactly how they were in the Genesis days (the best days for Sonic). It plays just like one of the old games, but there's whole new level of polish that makes it so great (60fps, better looking/better animated sprites, new carefully designed zones, AMAZING music, etc.)