You sort of had to be there at the time. It was an amazing experience that Nintendo has subsequently topped.
I came to it a couple years late but I don't think I was that far off it's release date.
Honestly the biggest thing for me with the Galaxy games is bouncing back and forth between them and NSMB Wii and just comparing the feel of jumping. In 2D Mario it's
fantastic, every hop just has a joy to it, I can land exactly where I want, I feel in control. In the Galaxy games it's just floaty and it's really hard to go where you want. It's a fundamental flaw in 3D Mario, made a lot better in 3D Land + where you're locked to 8-way movement with a better camera, but still never as good as 2D.
I found Galaxy 2 underwhelming after it. Yeah it was still fun and had more crazy levels, but the boring plain vanilla overworld, less coherent theming, and general lack of freshness, it really was a straight iteration of a concept I had already done.
The basic level structure of Galaxy 2 was my favorite improvement in that game. Everything about going back to that stupid hub was so annoying (and slow), and having unskippable cutscenes for the Rosalina parts in Galaxy 1 was just dumb.
The levels not being "fresh" is valid but they're just much more to the point. The whole game is pretty much a remix version of Galaxy 1, one which if it had come out instead in 2007 I would have much more favorable opinions of that game. But the levels just being way more to-the-point made the whole game feel much tighter, made me want to keep going in a way that Galaxy 1 didn't. Galaxy 1 often felt like a slog trying to get to the finish, whereas Galaxy 2 I didn't want to put it down. You could really see what they were moving towards with 3D Land in just getting rid of the illusion of open exploration and moving towards the 2D style of point to point levels.