This is so great! At first, I was a little confused and lost but quickly understood what was going on.
Dare I say it plays better than the original metroid (IMHO, Metroid only really becomes great with M2 on the original GB).
Gotta agree with Max on this, regarding the controls but I would also add that spotting the spikes and differentiating it from the "vegetation"(?) was a little hard. Perhaps adding some color-cycling to indicate it is damaging would help to convey this idea.