Needs some tweaks and polishing, but good attempt.
Normally I give up pretty early on most peoples side scrolling attempts, but I played through this one mostly until it locked up before Bowser. You did a good job on this. You've definitely got the spirit down. It's worth going back and fixing some of the details.
Things I noticed:
- Objects falling are very sluggish. They're like feathers falling to the ground. This applies to the player, the power-up shrooms, to the Goombas, etc. Might want to try for some more realistic falling. (Basically involves increasing the amount the object drops in the Y direction a little more each frame until you hit a certain maximum falling rate.)
- When the Hammer Brothers show up, I could barely see their hammers. It was like they were just flashing into existance. Luckly I was going at full speed and just bypassed them. ;)
- Definitely some hit detection issues. Several times I died in situations where I either should have simply passed by one of the fire wheels, or stomped an enemy. (Indeed, I quit the game after, as a little Mario, I stomped a Goomba someplace near (but not on) a firewheel and the combination caused it to lock up.)
- There's other things, but they're small. Oh wait, one I said I'd remember for the commentary: bricks should only break on the upward jump. (However, it did make for some awesome Goomba-head/brick combos...hmm, perhaps you're unintentionally on to something... :))
Good work, hope to see more.