Galaxy 2 is a remarkable title because it somehow snuck up on me and became my favorite 3D Mario of all time.
As noted in my thoughts on its predecessor, Galaxy 1's one real flaw to me is that its more open levels generally tended to not be as strong as its linear variations and often weren't as engaging as similar sandbox stages in something like 64 or Sunshine. SMG is still an incredible game; it just feels like it can't quite decide what its core identity is.
Galaxy 2 is a very thorough refinement of that, trimming the proverbial fat and ending up with some of the greatest set of linear stages in the history of the franchise (certainly in 3D). It's almost like an arcade Mario game; distilling level concepts down to their core and never really allowing them to overstay their welcome. Cosmic Clones are a great example of that, being challenging enough to force players to think about their movements while not being so common that you ever really get tired of them. Same thing with the timed comet levels, moving away from just a fairly pat time limit idea as seen in Galaxy 1 to the more organic concept of having to grab clocks to keep the timing from running out.
Gameplay wise, Yoshi is an enjoyable addition as well. While some fruit stages lean into being a bit gimmicky, he controls so well, and his tongue being mapped to the pointer feels so seamless that it really mitigates much of the issues with the former. Rock Mario is a decent addition as far as power-ups, but Cloud Mario is the real stand out, not only giving a floatier jump but really incentivizing creativity with the platforming via the cloud creation. Fluzzard is the only real weak new idea (and arguably the only real negative in the game) as it's a basic motion control flight mechanic that feels stiffer than what Twilight Princess had been doing with the same concept 4 years previously. Still though it's not in the game nearly enough to be that much of a downside.
I also think it's worth noting that even with its linear focus, Galaxy 2 does do exploration well. The comet medals are something one has to go out of their way to find at points and the post-game green stars are fun, often involving outside the box thinking to really find the later ones. In general, I'm reminded of exploration and replays were utilized in another game that came out that same year: Sonic Colors. That title's inclusion of Red Rings gave a nice bit of depth to its levels, and I'd argue SMG2 achieves the same thing with the comet medals in the main game. Both releases make going off the beaten path worthwhile without making the search too tedious.
Finally, I want to highlight the aesthetics. Galaxy 2 gets some criticism from moving away from the darker more majestic feel of the first entry, but for me it's a perfect blend of that game's sensibilities and the bright/colorful sense of the broader Mario series. Unlike 3D Land or 3D World, solid games that leaned so much into the series generic tropes that the identity of individual levels was lost, SMG2's stages are unique while being distinctly Mario. Sweet Mystery Galaxy might be why I love the look of this game so much; a deep and wonderous looking space background for a stage physically made entirely out of desserts. That is the essence of Super Mario Galaxy 2.
So yeah, overall, I adore this game. Even the weak final section of the last Bowser fight is made up for by the challenge of Grandmaster Galaxy and final test of the Perfect Run which will make you tear your hair out while being incredibly satisfying to finally beat. If you want the more emotional and atmospheric experience, it can't compete with Galaxy 1, but as a game proper it gets so many things right and is a masterclass in distilled platforming design.