You ripped off classic Mario, with a few minor alterations and removal of powerups. Not cool man.

I like it; I think it's going to be a great game, eventually.
I love how you recreated parts of Super Mario Bros levels. :D

Couple of things I wrote down:
– In the menu, going back from level select to the title screen with 'Esc' doesn't remove the text from the level select, so you see it overlapping
– Your movement, especially jumping, should be faster. It's great that there's different jump power depending on how long you hold the up arrow, but overall it's slow, so it feels like you're on the Moon. If you draw inspiration from Mario, check how fast the movement and his jump arch were in comparison.
– You look left/right when moving, but straight in front of you when jumping. You should also probably look left/right in the jumping animation, otherwise it looks like you're jumping sideways. Also no need to reset the player to the same "facing front" position whenever you stop. Just keep him looking left/right according to the direction you were going. In 3D games, you also wouldn't rotate the character around whenever you stop; no need to do it in 2D.
– The coin count number could be slightly bigger
– Fullscreen option? The game's resolution is pretty big, so being able to play it in fullscreen mode would be an additional benefit.
– Mario also had movement inertia (continue moving a *little* bit until you stop completely) and running. It's up to you whether you think your game could benefit from that.
– I'd pick a different colour for the flying enemy. When I saw him for the first time, I thought he was a collectable item or some sort of bonus, since he's gold and has wings.
– The controls are very reliable and feel alright – thanks! Plenty of platformer games sometimes refuse to jump when you need to but it never happened to me in your game.
– Punching a "?" block sometimes moves it slightly below its original position.
– Mouse clicks move the character to the left. Uh… why?

It's a very good prototype. Good luck with the rest of the game!