I think TYP was probably referring to me on the podcast when he said that racing through the game is not the way to play it and that perhaps this game wasn't for me. Certainly my feelings were confirmed last night when I started playing the Lost Levels and absolutely loved the simplicity and challenge of it. The screen never scrolls up or down so there's never any ambiguity about what the player could be missing above or below (the exception being vines that take you up, but these bonus segments are wholly unavailable without the necessary block breakage down below!). There are far more possibilities in SMW, with far greater variance in how high and far Mario can jump and other additional abilities. This added depth doesn't appeal to me.
Now as many people have pointed out, broadly speaking you have the more linear driven games - SMB1-3, and Galaxy, which retreads this route a lot too - and the not so linear, e.g. SMW, SM64, Sunshine - but a key distinguishing factor between members of the same group is of course, an extra dimension.
For me SMW doesn't work simply because of the limits of 2D. The unforgiving nature of exploring - not being able to see what it is below you is often a frustrating experience which has lead to many a death on my part - especially when a great number of the bonuses are high up, above the regular stage. The short supply of health and more pertinently, power-ups like the cape makes exploration an experience that requires a lot of patience. This is not to say that I don't like SMW, it is a decent game and I did play all the way through, and got most of the way into the Special Levels, which I must say, along with Star Road is an extremely enticing prospect for budding players - not just one, but two secret worlds. Regardless those special levels can be infuriatingly difficult and I still haven't beaten them all. This is pure preference, however even these levels, being far more restricted and aimed at pure platforming, aren't anywhere near as tight as I would like due to Mario's slippery footwork and physics.
Another annoyance, and this is also present in Donkey Kong Country 2, is that going back to levels you've already done to quickly grab something and then exiting out with select is such a chore. In DKC2 it was to nab coins with which to save or hop on an aeroplane between worlds, and in SMW it's to grab a cape or Yoshi, and preferably a backup item in the box too, in order to protect yourself to get past a difficult level.
Back on point, I realise now that this was a highly ambitious game at the time, moreso than I had previously given it credit for, and yet for me SM64 represents the realisation of the ambition SMW aspired to, which was to have a exploratory, more adventure-y platformer with the key principle of multiple secret 'exits', which SM64 evolved into a series of different goals, all within the same (albeit dynamic) stage. Fortunately in 3D the levels go from being something you run into blindly sideways to being literally in front of you. There's also the advent of a health bar and losing health never limits Mario's physical ability, so it's far more accessible in that sense.
Anyway I had to get this off my chest as it goes some way to explaining my lack of enjoyment from this game. Oh yeah and the music is by far and away the worst in the entire Mario series, which doesn't help proceedings either! This is a long way from a bad game though, don't get me wrong.