Jump on Mushrooms: The Game (JoMG) is one of those games which don't come along that often, and when they do, they reassure you that somewhere, someone is still squeezing more life out of the platformer genre with new, innovative ideas like this.
In the case of JoMG, rather than running through the level collecting coins and stomping on enemies, you're playing in reverse, dropping coins and sending stomped enemies back to their previously stomp-less state. It takes a fair bit of getting used to and for a while you're just going to die until you can get into the right state of mind.
The ways that you can die are basically just the results of getting into a place that you wouldn't be able to if you had played the game normally. An example of this would be un-collecting more than the number of coins that you'd been given at the start/end of the game. If you end up with more coins than you collected on the way there, then the game rewinds itself back to where you began/ended. Needless to say, it's all a little confusing, and really difficult to explain.
Sound wise, it's exactly as aurally horrific as the Mario theme tunes being played backwards can sound and you'd do well to mute your speakers in preparation for the explosion of reverse monophonic noise.
Graphically, it's like Mario, except slightly less adorable than the old, chubby Italian plumber we're all used to, but everything's readily identifiable and that's generally the point of graphics.
To sum up, JoMG is Mario backwards and put through a slightly horrifying filter. You'll play it and you'll enjoy it, but you're not going to play it all that long because it is so utterly devoid of polish.