The camera sucks insofar as you can't make a quick 90° turn, either the camera will adjust automatically, rotate your direction of movement and let you drop (there was one "secret" that was damn annoying with this) or you take too long which is fatal when you're on dissolving ground. I preferred the camera in Wario World's platforming bits, at least it always aligned with the level and when you have to make a quick turn it wouldn't rotate and screw up your movement.
Another, MAJOR annoyance with SMS was that there were very few "checkpoints". Feels like SMB3 all over again (my least favourite 2d Mario). Make your way through a long level, die shortly before the end and restart from the beginning (at least SMB3 had very short levels so it was less of an issue). What is this, 1985? Even SMB1 had midlevel checkpoints. It's especially annoying with the "secrets" as dying is pretty easy in those.
The lives in that game were rather useless, most of the levels had no checkpoints at all so when you went Game Over that didn't mean jack. It only meant something with the secrets and there only that you didn't have to play the "exposition" again, you'd have to restart at the beginning of the secret, anyway.
They should group sets of levels (2-3) together and give them midlevel checkpoints. Let the player only choose the group, he has to finish it with one set of lives. Run out, restart the group.
M64 had too many moves, especially lots of redundant ones (sideflip, backflip, longjump, slide, ... Who needs that many? There was a time when the jump and the dash jump were enough). SMS administered the necessary cleanup, SMW style there.