Turns out, I actually like this despite the gripes I have with it.
1. Gravity is too slow. Falling speed should increase over time.
2. The "Flip" blocks won't reset after each world.
2. The hit detection when you jump is too big. Many times I would try and jump onto a platform, but hit my head and fall instead. The large hitbox wouldn't be much of an issue if it weren't for...
3. You HAVE to hit an enemy right below where they are. what's the point of having the three tiles bump if the two outermost tiles won't hit the enemies.
Honestly, if these issues were fixed, and the game slightly polished, you could have a pretty fun clone of Mario Bros. Heck, maybe even make an adventure out of this where each world presents new mechanics or something.
And yes, I know this is your first game (or one of), but I suggest at least trying to fix some issues so you learn what can work, and what won't.
That said, I'm just some guy that played this, so I don't expect you to listen.