Gotta say a couple things because it's the kind of game I've been asking for for a while.
It's just a real gamer's game. If you're in your 30s you may recall the 16-bit era as the pinnacle of skill based gaming. Innovation was king, brand new genres were being born and refined, 3D and complex controllers had not yet bifurcated the market, the graphics were lovingly crafted pixel by pixel, frame rates were smooth as silk. Most importantly, the 8-bit mentality of making every level count still reigned, you had to earn your passage by learning new mechanics, working on your reflexes, and praying to the heavens to let you progress.
That's the kind of game this is. It asks so much from you, but gives it all back and then some. New twist on gravity, jumping, shooting, level design. It's all the stuff we used to expect from a new game; the stuff that actually makes you feel like you accomplished something when you beat it, that you took on a new obstacle and conquered it.
Maybe I've missed all the brilliant paltformers and shooters that play at this level. I've heard similar things about Super Meatboy and smaller PC indy games, and some Nintendo platformers still have some of it, but personally, this is the first game I can recall that captures the best of the 90s hardcore mentality since the 90s. It's simply the coolest of the cool, if there is anything else that recreates this feeling please let me know, would love to try it.