Woah, thanks for all these positive comments! Let's see about some points you guys raised:
1) Invisible walls/square not expanding: That is strange, I have played the game multiple times on all three of the major browsers as well as the Windows version, even after you pointed it out to me and I could not reproduce it. I would be very happy if someone could provide me with a screenshot and perhaps details on their browser, but since it all works for me, I imagine it'll be really hard to debug :( I'll see what I can do about it though after some sleep.
2) Walljump: You just need to touch the wall and press "Up". No need to press any directional keys at the same time. I can see though, how the walljump in addition with other moves required to pass hard parts can get a bit troublesome, but that's more or less intended. I will keep this in mind though. If I keep working on the game for a post-LD release, I'll see what I can do to improve the controls.
3) Painful sound effects: I put these together with bfxr, and some came out a bit more annoying than I wanted them. Sorry about that :( I did add an option to mute the sound effects though (press M).
4) Sound problems: I think this may be some issue in Game Maker Studio's HTML5 export. As everything worked well for me in 3 browsers I couldn't really do much about it. I'll do some research though to see if there are some GMS known bugs with audio that I can work around.
5) Crash: Thanks for the report, Phaax. Did you try using another browser or the Windows version?
Once again, thank you all for the comments, both the constructively critical ones as well as the really positive ones. It's super late, but once I got a bit of sleep, I'll go play all your games :)
Good night and have fun everyone!