- Make "jump" open the next level instead of having to click. (It is annoying to have to switch from keyboard to mouse and back, especially if - later on - players will die a lot from your traps)
- The gravity and movement speed can be higher (it feels like your game is playing on the moon at the moment). Adapt the physics NOW, BEFORE you create more levels, otherwise you will later have to remake all the levels because of the changed physics!
- this type of game has been around since forever. Good job as a first attempt (you got something together, brought it to the page and we can actually play), but here is a hell of a lot to do: What will make your game unique? (Collectibles? Storyline? Powerups? (A hint would be a hook to shoot to ceiling and hang from it, wall-jumping, double jump ....) Enemies? Can the cube shoot/fight? (you can replace the cube with any character, give him a sword or bow and arrow, or a machinegun ...)