There are some aspects of the first that weren't strictly Mario that I liked better. The epic, space-opera-esque opening and ending sequences and cutscenes, for instance. And while it gets a lot of hate, the hub world slowing you down made the game feel "deeper." That's not defendable to anyone, but it's just something I liked.
I will say I like Starship Mario just as much as the Observatory, but for different reasons. And the level progression is better, because it's from past Mario games.
There are also some other little nitpicks I have that weight the game down lower than the first for me, but they're just my opinion.
To me, SMG2 doesn't do anything crazy awesome to wow me any more than the first did, the level design in both are just about equally awesome, so because of that, the smaller things are what make or break the experience for me.