By the same token though Nintendo hasn't really done that outside of a few Star Coin areas and - if their internal data reflects that my friends'/family's performance is typical - I suspect they never will get too advanced with it. The attraction of 2D Mario lies in part on its simplicity: you may be called on to almost master some of the techniques, but there are only a handful of said techniques to master, with level design and enemy placement introducing the vast majority of the games' variety. The more layers you add on, the more danger there is of turning some folks off. I don't think it's an insurmountable barrier, but Nintendo has had four bites at the apple and still refused to swallow.
Yes, but that has more to do with Nintendo's general attitude of shifting the difficult parts into the postgame content; and what they did was not making levels designed around the wall jump, but levels where the wall jump becomes more or less useless (doesn't go for all levels, because there are also vertical levels in the postgame worlds). The difficulty you find in SMB3 from world 6 onwards was basically relegated to a few entirely optional levels in the NSMB games.