I liked the overall idea of this game. In some ways, it was a twist of sorts on video games that occasionally reverse your controls; for instance, I couldn't help but reminisce about the final boss fight of Donkey Kong Country 2, my first experience with such an inversion. And you're right about the comparison to, say, learning a musical instrument, as this felt very similar in many respects. In terms of the design itself, the graphics were simple and served their purpose well, and the basic concept was pretty well-designed and artfully executed as a whole.
Unfortunately, there were a fair number of substantial bugs in this game. As a simple example, if players were a little bit slow (as I was), the feet shooter levels sometimes devolved into pushing enemies off their paths rather than actually shooting them. Considering the irregular manner in which the enemies moved and spun after hitting one of the feet, this seemed like a bug rather than a gameplay feature.
Much more importantly, a few levels did not seem to properly recognize having been completed. This was the case for one of the feet levels as well as the second brain challenge. The former instance seemed to be one in which the game simply stopped midway through the "level complete" animation, and it was easily fixed by refreshing the page and replaying the level (as well as, unfortunately, several that preceded it; the game did not appear to have saved after those levels were cleared). The latter case, however, was intractable. For this level, the game sometimes seemed to recognize the left side as having been completed based on the thumbs-up that appeared, but never the right side. Oddly, this happened even when I deliberately completed the right side without moving the left. (And yes, I realized that the brains and cookies needed to simultaneously reach their destinations in this level, but even moving all four to their objectives at the exact same moment -- or as close as possible to simultaneous left and right movements -- didn't result in level completion.) Neither reloading the level within the game nor refreshing the webpage itself solved the problem, so this ultimately made it impossible to progress through the rest of the game. I'm not sure if others have encountered similarly unfixable errors, but that was a rather devastating one for me.
Beyond this, it might have been useful to more clearly indicate the effect of certain game features rather than to rely on self-discovery. I struggled for awhile with one of the levels featuring TNT blocks until I realized that it would set off a chain reaction rather than a one-time explosion, thereby making the puzzle simple. Tasks like "BRAINS ON B COOKIES ON C" probably could have used a more complete explanation as well. I'm still not sure if there was merely some hidden requirement that I failed to satisfy, and it's a problem if you leave players wondering whether their failure to complete a level resulted from their own performances or a glitch within the game.
On a separate note, from the 30 or so levels that I was able to play, there only seemed to be two primary challenges: mirrored puzzles and speed runs. I can't speak to the remaining levels, but it might have been nice to push players' ambidexterity in other ways. I could envision, for example, levels focusing more on timing or on using more delicate, precise movements than those required of a grid movement system. For a game that supposedly emphasized ambidexterity, it was a little odd that the most difficult task on so many levels was puzzle solving rather than coordinated movement. (In fact, many of the levels could have been cleared by completing one side at a time rather than engaging in simultaneous WASD and ↑←↓→ movement, which kind of defeats the intended purpose.) Perhaps such level design diversity would have been too much to ask for an 8-bit style game, particularly if it would require going beyond the grid movement that formed the core of this particular game, but it's a thought, at least.
To be honest, most of my critiques are quite nitpicky. Aside from the outright bugs (especially those that kept levels from registering as complete), I really enjoyed this submission and its relatively unique concept. It's just a shame that these glitches hampered the final product to such a degree that the game could not be finished, as it was otherwise very cute and fun. I liked how lighthearted the game was, and it worked very well as a low-stress challenge. Fix the bugs, and you could really have a solid game.