For me, at least, it's not about the individual stars so much as the order in which you get them. In the first couple of Mario's 3D outings, it was possible to jump into a level for the very first time and come back out with the sixth star for that level. Now it's basically "get this star, then this star, then this star," etc. I think that this, in turn, has led to the
courses feeling more railroaded, with no sidepaths and no real sense of freedom. While the methods to obtain the stars have always been more or less set in stone, the courses in
64 and
Sunshine were much more open to exploration and experimentation - contrast with the
Galaxy games' levels, most of which consisted of a series of platforms in a rigid order that you had to jump across to get the star at the end.
Now, I don't necessarily agree that a 3D game should always be a massive, open world affair, but the more explorable and nonlinear format has always worked better with 3D Mario.