The Super Mario franchise has been known for simultaneously advancing the medium of games while still being inviting to new players. It did that with the original Super Mario Bros. when it popularized the sidescroller, even more so when it redefined the genre with Super Mario Bros. 3, and Super Mario 64 exemplified exactly how to make platformers in 3D. When the Wii hit the scene, that system needed a whole new Mario game to evolve the genre once more. Fortunately the Super Mario Galaxy games were up to the task, and Super Mario Galaxy 2 in particular marks a highpoint for this generation in terms of design, creativity, and outright fun.
Though a handful of games like Tomb Raider and Ratchet and Clank took 3D platforming in new directions, the genre still had many of the same problems that had been around since Super Mario 64. It was difficult to judge the distance of jumps in three dimensions, camera controls were wonky, and most games relied on reusing the same levels with different objectives. Super Mario Galaxy 2 addressed all those with such ease that it's amazing that they were ever a problem.
The dramatic shift was in the level design. Mario still controlled the way he always had in 3D games, but the new galactic setting of exploring a collection of tiny planetoids refocused the games. Instead of dropping players in a big open world, each tiny planet had a clear objective and easily defined space for the player to explore.
Since each planet had its own clearly defined gravity, platforming was much easier to understand in three dimensions. It also helped that planets came in such varied shapes that the devs could place the camera in much smarter ways, ridding players of the hassle of carefully tweaking the camera so as to best judge a stage. And even with that clarity of design, the stages were filled with little details that made each visually distinct.
Though clarity is one of Super Mario Galaxy 2’s greatest strengths, so is variety. It rejects the annoying trope of searching for countless collectibles in the corners of a handful of large areas. Instead, SMG2 kept finding new ways to entertain with each objective it threw at you, never bogging players down. Innovative areas and enemies were everywhere, and in the rare cases when the game reused something, it was always for a clever reason.