Nice! Graphics/music is good, gameplay is like SMB, but that's what you went for so it's ok.
The one problem I had (at least in the demo) is responsiveness of controls. Maybe it's just specific for me but it seemed like everything was lagging. e.g. when I run and jump (in the level with big straight run followed by big jump), I have to press jump almost a full screen earlier than the edge to actually manage to jump and not just fall straight into the spikes.
When I actually do jump (a bit too early was the case) the graphic shows me going straight for the saw, and then as I hit it I am magically transported up onto the safe ground as if I jumped the pit properly.
I noticed this on many occasions - I tend to fall into a pit before actually reaching the edge of it, and jump across one safely even when it is obvious that I shouldn't (that I was too short with my jump).
Maybe it's a glitch on a slower computer/browser (I was on Chrome, but my comp here is nothing much), but if it were just laggy graphics my jumping trajectory would still be proper, just with skipped frames. Instead, it felt "moved", as if my actual jump was always a bit (sometimes a big bit) shifted in comparison to what the graphics showed.
Again, if it's just me, don't worry, but if others see it too it might be worth testing a bit on a lower performance machine.