And I still find it hilarious that people think Super Mario Galaxy 2 is really a new game. Sonic tries to appeal to its audience by trying to do new things. Mario sticks with the old and does nothing different, if you haven't realised.
I really don't think anything could be further from the truth. Mario has done massive sweeping changes in its gameplay over the years, completely changing to suit new technology. He went from defining the 2D platformer to inventing map screens, creating the entire concept of a hub world and an exploration-based 3D platformer, and then perfected the obstacle course style of 3D platformer...in space.
I suspect you just don't think about these changes because they happened naturally and unlike Sonic's massive changes over the years, were actually well thought out and well executed. Mario never had any physical attacks in the 2D titles, but then in Super Mario 64 they added a huge variety of punches, kicks, slides, and a massive variety of different jumps. They also removed large parts of the classic Mario gameplay, such as breaking blocks as a gameplay mechanic. Instead of trying to force the original Mario design into three dimensions (which wouldn't have worked), they came up with a NEW design with a new overall vision, and then built a game around it. To be honest, the only real thing that connects SM64 to SMW is the character Mario and the creative theme of the game world. The gameplay itself is completely different. Also, Super Mario Galaxy changes everything yet again, because while SM64 uses exploration as the central gameplay mechanic, SMG returns to a more classic platformer design of challenging the player to complete obstacle courses.
Now look at Sonic's transition into 3D. Classic Sonic gameplay, like classic Mario gameplay, doesn't really work in 3D, so they changed the gameplay in Sonic Adventure 1 to...well, just about everything they could think of. There is basically no overall vision to SA1; instead, it's a giant mess of what seems to be every idea the development team had, ranging from linear races to a rail shooter to a fishing minigame. This just doesn't work, and generates a game that by design feels schizophrenic. As they've moved on from SA1, they've continued to make the same mistake over and over and over again, coming up with gimmicky idea after gimmicky idea to get the attention of players, while ignoring the problem that there really isn't any substance underneath any of it. SA2 continues to have multiple play styles while only adding polish to small segments of it. Sonic Heroes has multiple play styles that you constantly have to switch between (but don't really add anything except "use mode X in situation Y"). Sonic 2K6...well you get the damn point.