Hard to tell from the video, and it has been ages since i tried Supertux, but all too often when i try SMB clones they have this slippery feel to them. That when i let go of a directional button the character do not stop instantly, but "slides" for a block or two before coming to a halt.
Isn't that part of the later SMBs? It's not in Super Mario 1 + 2 for the GB, but sure is in SMB3 (although I only played the latter on emulators only).
I'm pretty sure it's part of all of them. At least, I just tried it in an emulator and SMB definitely seems to have something like inertia built into the walk speed, and definitely running.
Possibly, but i don't think it is as pronounced as in the various clones. Or at least i never found it annoying in the SMBs, but kept noticing it in the clones.
Supertux definitely does have that effect (I hate it ;). I only played the original Mario in emulators, so I can't tell you whether it's copied from original.
SMB3 has some sliding, more if you are on ice surface. Supertux has a wee bit less than SMB3 on ice surface, but all the time. I guess that´s intended as, well, you are a penguin on an icy world! :)