Nice idea and implementation, I like it.
I agree with the commet about having some level chooser: it would be nice to return to the game and start from the point you've finished at. Besides, it would be nice to track your overall progress.
Also, it would be good to have some calm, simple and unobtrusive background music performed it the same 8-bit / chiptune genre. I think that something like "Lode Runner" (NES) and "Nuts and Milk" (NES) have would fit "Upward". Appropriate music can make really great atmosphere in your game; moreover, it will continue playing in heads of people after playing the game (people will just associate game with its music).
And one more little remark: for less experienced players (like children or just casual players) it would be nice to have not so challenging levels at start: players feel more comfortable if game difficulty grows smoothly.