Hardcore
In a time where you can't play a game without getting achievements thrown at you, finally there's a game where I'll need to grit my teeth and buckle down in order to make some progress. I may not be a 'hardcore' game player (I don't mind achievements thrown at me), the idea of memorizing the sections and skillfully making my way through is very enticing.
Everything in the game is part of the puzzle that makes it work. Having one life means you need to learn the routes; the sound gives it a frantic feel; the moving wall chasing you gives a rush and the stylish pixel graphics remind me of a more simpler time in gaming.
It's not without it's imperfections, though. Although it strongly emulates earlier video games, I shouldn't feel as blocky as I did in those. When jumping and especially climbing on ladders, I sometimes feel I could end up dying through no fault of my own. Apart from that though, it's a really good game.