Like it! Great effort for 3 hours. The instant death is an awesome feature in this format and can be expanded so much (parts mentioned in the missing-features).
I do feel the game with the instant-death features of ONE mistake lends itself better for a more puzzle-like play-style, where every step needs to be calculated, however the long level with just makes you want to walk FASTER and mistakes instead happen basically just because of impatience instead of a planning mistake.
I feel more effort could have been spent on getting one enemy or another puzzle mechanic to flesh out the gameplay and leaving the level more or less just screen-sized.