Well, yes. There is a different feel to using a d-pad and using four distinct keys, and it throws me off when I'm playing a 2D platformer. In SMB, the inertia feels totally wrong using the arrow keys or WASD.

I agree, but I think it's because a d-pad does two things: 1) it lets you press two directions at once with a single finger (all four diagonals are actually two presses, e.g. up+left); 2) it stops you from pressing two opposing directions at the same time like you can on a keyboard, e.g. up+down.

Once you get used to these limitations or find a game where diagonal movement isn't required (not sure about SMB), it's really quite tolerable to play a 2D platformer with WASD or arrow keys. But you're still right it doesn't quite have that magical kinesthetic quality that a d-pad does, and I'll always choose a gamepad myself.

Not easy to play on a keyboard using only two thumbs.