The NSMB games have different physics.  Also, Mario moves much faster and tighter in Super Mario World than in the NSMB games.  The controls are a lot more sensitive.  So it'll take some getting used to.

Hard to play as in: NSMB is not as good compared to MWorld or because you think it is harder? NSMBds was a walk in the park for me. Apart from some coins (or were they medals?) which kind of demand some thinking in order to find, the game by itself was very easy. Too easy and accessible in my opinion.

Getting used to physics again. Trajectory and whatnot.

Forward, to the back, above, below, you won't have much trouble. lol In NSMBds the character feels more loose indeed, I guess it will be easier though. Then they added the wall jump to that game and even falling on pits seems like a joke after that.

There's a fanmade game around using that engine for the DS. That game is quite challenging and fun. Maybe you could add it to your to play list. Never had the chance to finish it.

who the heck try to play this kind of games with joysticks? kids this days!

lol yeah, so true. You need the precision of directional buttons for 2D platformers.

Always use d-pad for 2D games, fighting games and anything that requires fast inputs because d-pad is faster than analog.

Maybe your TV has some input lag? We know that the gamepad screen has very low input lag (~1 frame).

Well actually, LittleBigPlanet works rather well with the analog sticks, but that may be because of the way that game in specific plays, feels and reacts, with that kind of heavy and loose physics.

Super Mario World was the first 2D Mario game I ever played, and I didn't find it too difficult at all, only uber funs. :)