Not bad for a first crack, I need some closure
Marios reactions are done really well, there aren't parts where he looks like he's falling with strings attached or anything, and all the sprites were clean cut. My beef, it needs an ending or something. They kill him, they blow up, they get married and have kids, anything.
I don't know much about faking slow mo other than just doing it by hand but if you take all your frames and turn them into one movie clip, you can zoom in and out how ever the hell you like with out having to redo animation.
Keep at it.