This certainly feels like an expansion pack, and given the development of the game it makes sense. Essentially the dev team had more ideas for levels and they decided there was enough here to make it a new game. While the level design is good and highly polished, the story itself is very bland and essentially a complete rehash from Galaxy 1, just with much lower effort. I don't play Nintendo games for the story (honestly Nintendo is pretty bad when it comes to storytelling, in general of course). I come for the gameplay and that's something Nintendo always excels at and that's no different here. It's no Majora's Mask, but is ironically what people who played Majora's Mask back in the day expected and probably wanted. Not me personally, I like the experimental weirdness of Majora's Mask. Making it one of the most fascinating sequels ever made by Nintendo.
This game doesn't push the envelope at all, but it adds refinement and some new fun mechanics. There are still aspects of this game that feels half-baked, such as the Boo power-up being severally underutilized (only present in a handful of worlds and even then it's not the main focus). Plenty of power-ups also feel redundant. There is overlap between the Bee power up and the Boo power up for example. The rolling power up isn't bad but also feels a bit odd as an inclusion. None of the levels with the roll power up feature much platforming. The only close platforming challenge with this power-up is the bowling star, which is fun mind you. But the rolling power up doesn't really compliment the overall design and mechanics of Galaxy 2.
That said, I do really like the cloud power up. I like the concept of it having 3 charges and the balance around additional cloud pick ups is well designed. Another great thing about the cloud power-up is how it's so focused on platforming. While the Bee and Boo power up do help with platforming, I'm not really a fan of either. Since both have you fly, it actually just completely by-passes the platforming. In sharp contrast, the cloud power up still requires you to platform, you just get a safe guard by having control over making your own platform. One of the optional post-game worlds is a platform challenge with the cloud power up. It wasn't particularly hard, and I honestly was able to beat it without ever using the cloud power up, which was a nice design. The cloud power up functioned as a safety net you had control over.
Yoshi is back and it is probably the best part of Galaxy 2. Yoshi is finally an actual power-up again as useful, in fact more useful, as he was in Super Mario World. Yoshi even gets his own power-ups that are fun to utilize. There aren't a lot mind you. But they push these in interesting challenges and Yoshi importantly feels good to use.
Another big shift in the game is the change to the hub of the game. I liked Mario Galaxy's hub, so I am a bit disappointed to see it go. What we get instead is a ship, that is honestly pretty pointless, and a game board grid similar to Super Mario 3 from the NES. This creates an unfortunate linearity to game progression, in sharp contrast to Galaxy 1. Galaxy 1 had a degree of linearity, but you could at least access multiple worlds and complete some stars in different order. There's some branching in Galaxy 2, but another unfortunate game design decision was artificially locking most stars behind progression through the main story. Mario 64 still had the best design on this front. Where you can enter a world and get all stars before moving on. In galaxy 2, you can get 2 stars, then you need to progress further into the game and eventually you will unlock more stars in this world. Maybe people like this? It personally led me to not bother going back to get more stars.
With that increased linearity, which I don't like, you do get a lot of variety in the levels. They throw a bunch of different themes at you regularly. And it is legit impressive how elaborate these worlds can be, even when there's only a handful of stars to get. That said, I have a harder time recollecting specific levels. It never felt monotonous but also never as memorable as Galaxy 1.
This is a fun expansion pack that presents a very refined gameplay experience. It has it's issues but was nonetheless enjoyable to play. And Yoshi was awesome!