Sorry Yahtzee but this is all nonsense. Yoshi's Island is an amazing game.
See, Yoshi's Island introduced (to the core Mario series at least) the notion that there could be tiers of success. There was none of that in Super Mario World. You could end a level as tiny Mario, you could end it with two power-ups, a third in reserve, and a Yoshi standing proud and erect between your legs, but the game called that the same thing either way. A tick was a tick. That level was done, move on. Often the level wouldn't actually be done because there were secret paths and keyholes we could have taken, but the game would have the decency not to hang that over us, making the accurate assumption that a lot of us couldn't give a shit.
Yeah, no. This is exactly the same thing. Either you care about getting everything or you don't. If you care about getting everything, you want to get all the secret paths so the level doesn't glow red on the world map anymore and you see all the levels. In Yoshi's Island, if you care about getting everything, you get a perfect in each level so you can unlock the bonus stage.
Yoshi's Island was much more about challenge, a challenge of how much you want to put into it is how much you get out of it, which is why all the secret levels that get unlocked are the hard as nails ones. The game knows the people who want the challenge are the people who want to get everything, and not the people who blow through the levels not caring that they missed a flower.
There's plenty of little sublevels in the game whose only purpose is a short challenge course to get some of the red coins or flowers. These serve no purpose towards beating the level, they're ONLY there to challenge the people who want to get everything. This kind of design is not bad - it really WAS ahead of its time.
You could also deduce the presence of a secret in Super Mario World, if an area has a suspicious lack of a ceiling that might imply a hidden route up, or if you saw a keyhole missing a key.
If it weren't for Nintendo Power, would people REALLY have figured out that in Mario 3 if you duck for 5 seconds on white blocks that you fall behind the foreground layer? I mean, sure, someone would have eventually, but let's not pretend previous Mario games were so well designed in the secrets department. I think my favorite one in Super Mario World is the one where you have to go UNDER an end of level gate either by a crazy dive bomb with the cape or a suicide Yoshi. Good luck figuring that one out naturally!