The thing about Mario is you've got options. I don't have to use the cape, and the levels are still great.
SMW's levels aren't that great. Hazards are easy to bypass and there's no real difficulty until you open up the Special World. There are also so many power-ups that, combined with the fact that you can store a power-up, losing them carries virtually no risk (though you won't lose them very often because the game doesn't offer much of a challenge).
I do think that SMW's levels tend to be pretty wide open, with most of the challenge coming from exploring to find alternate exits more than anything. Other than maybe Ghost Ship and Tubular, I can't think of an actual level that puts up much of a challenge. Maybe doing the cape bounce under the goal gate in that one level to get to the secret goal gate and the three-up moon is a challenge, but I got that pretty easily because I cape bounced through everything else routinely because the game didn't design a number of levels to discourage me.
SMB 3 gives you a flying meter so that you have to economize your flight and really make the most of it, and I'm not sure why they took it out of SMW and let you stay in the sky forever if you want.
If you were going to argue that SMB 3 is better than Sonic 1 (or even 2 or 3+K), I think that's a viewpoint that I totally understand even if I disagree with it. And I'm not saying that SMW is a bad game. As someone who likes exploration more than almost anything else in his platformers, I dig that part of SMW. There are quite a few good hidden exits with only a few stinkers (like the key where you have to guide some coins-that-will-become-blocks up to a ledge that you can't even see with pixel-perfect accuracy).
But honestly, as a platformer, it pales in comparison to every other big-name platformer from that era, especially Sonic 2 and 3+K where the Spin Dash mechanic opens up the game. Sonic levels with their vertical branching are also designed far better than the levels in SMW.
I probably shouldn't have made this another Sonic vs. SMW thread, but you really emphasized that the game is so good as a platformer by using that interjection, and it's not even the best platformer with the Super Mario World name. Sonic is just so much more interesting in its level design, I think. Then again, I think that classic Sonic's level design is the best level design in a platformer straight up, so I clearly have a strong bias toward it.
One of my three favorite games of all-time is NiGHTS, in which Yuji Naka also uses the concept of vertical branching to offer the player options as to how to navigate toward the exit, and so I'm just a sucker for that type of design (and for Yuji Naka - I need to get a copy of Rodea the Sky Soldier, but I digress).