Yeah, I don't think it's necessarily a bad thing that Mario World relied on exploration. Goodness knows, it was fun finding all the secrets the first time through. However that does serve to diminish the replayablity of it. My main problem with the emphasis on secrets and hunting down goals is that there is no reward for doing so, other than having a little star next to *96* and turning all the koopas in the game into...Mario-like things.
It would have been nice to be given access to something a little more substantial for all your hard efforts.
Actually, Yoshi's Island and Mario 64 had this problem, too. The reward for all your collecting never really pays off enough.
In mario 3, there really is no need to collect or seek and search down anything because the game is designed with basic platforming in mind. The experience playing the game and ultimately the ending IS the reward. In Mario World, it often feels more like work a lot of the time, after you've already memorized where everything is.