Ah, I see what you mean. I can see two sides to that, though. On the positive side, it means that the level has something new and different about it each time you enter it, so you're not always trekking through the same area for every star. But on the other hand, as you say, it removes that exploration element, except for the one or two token hidden stars that appear in most levels.
The only star you could grab in Super Mario 64 without exiting the stage is the 100-coin star. I'm not sure if Sunshine had any like that, I don't recall it having any but I also did not collect 120 shines in it.
As for my own thoughts, I passed on the game at release since I thought the waterpack and the setting were lame. Now, I know a lot of people have said the same thing, but this is coming from someone who likes to clean, so that's really saying something. I didn't buy the game until it reached Player's Choice status, and although I enjoyed it more than I thought I would, I don't regret the wait.
I liked the worlds and waterpack interactions, and the little pure platforming segments were a nice change of pace. But there were signs that the game was rushed, one of which nearly ruined the game. I felt that the number of worlds was lacking, especially with the little variety in settings, but that on its own is manageable. The blue coins, however, were absolutely tedious, brutal, and plain not fun. Not a single one of them. They're the main reason I have never and will never get 120 shines, because it's just too painful and I refuse to spend so much time on something so unenjoyable.