I still think Super Mario Galaxy 2 is, more than anything, a "bridge" game. It led Mario into the more linear structure of the Super Mario 3D games (the director is the same, actually). More and more linear galaxies were added, with Cosmic Cove Galaxy and Starshine Galaxy seeming more like the exception rather than the rule - a process started with Super Mario Galaxy but that here started reaching pathological levels, in my opinion -, you could see that the story started losing importance and even the settings started losing relevance - I don't think the Star Festival venue was as memorable, same for the first planet you reached in Super Mario Galaxy 2 with respect to the one in Super Mario Galaxy, and I don't remember something like the twin stars exchanging lava of Melty Molten Galaxy in Super Mario Galaxy 2, either.
I'd say that Galaxy 2 started being focused on a simpler gameplay, while Galaxy was still focused on the so called "user experience" - the sense that you're part of something big, revolutionary, memorable, epic in scale, with a story that apparently takes Mario games to the next level, even though it's probably just Super Mario 64 with new gravity mechanics, some gameplay tweaks, slightly more linear structure, a less vast hub world, more linear levels and more cutscenes, dialogues and storytelling (and, of course, a moveset with many missing moves, sadly).
If anything, the Throwback Galaxy in Super Mario Galaxy 2 showed clearly how far the level design of SMG2 was from the level design of SM64, and I still think that galaxy was not a smart move - it was wonderful playing it, but then I got back to the rest and had trouble accepting the linear galaxies. Also, when I heard Good Egg Galaxy's soundtrack in Grandmaster Galaxy, I immediately thought about the obviosuly better experience of the first Super Mario Galaxy, where I was thrown into this strange yet recognizable two-sided planet, with Mario's world on the background, plenty of Star Bits falling on the planet, an epic music, while in Grandmaster Galaxy I was facing yet another annoying gimmick...
I don't think Super Mario Galaxy 2 is a bad game, I just think it's the game that made the transition to the Super Mario 3D games - in which ironically Yoshi is nowhere to be seen.
I'd say that Galaxy 2 started being focused on a simpler gameplay, while Galaxy was still focused on the so called "user experience" - the sense that you're part of something big, revolutionary, memorable, epic in scale, with a story that apparently takes Mario games to the next level, even though it's probably just Super Mario 64 with new gravity mechanics, some gameplay tweaks, slightly more linear structure, a less vast hub world, more linear levels and more cutscenes, dialogues and storytelling (and, of course, a moveset with many missing moves, sadly).
If anything, the Throwback Galaxy in Super Mario Galaxy 2 showed clearly how far the level design of SMG2 was from the level design of SM64, and I still think that galaxy was not a smart move - it was wonderful playing it, but then I got back to the rest and had trouble accepting the linear galaxies. Also, when I heard Good Egg Galaxy's soundtrack in Grandmaster Galaxy, I immediately thought about the obviosuly better experience of the first Super Mario Galaxy, where I was thrown into this strange yet recognizable two-sided planet, with Mario's world on the background, plenty of Star Bits falling on the planet, an epic music, while in Grandmaster Galaxy I was facing yet another annoying gimmick...
I don't think Super Mario Galaxy 2 is a bad game, I just think it's the game that made the transition to the Super Mario 3D games - in which ironically Yoshi is nowhere to be seen.