One Finger Death Punch seems like a quick and dirty flash game that should be fun for about 10 min and be over in 20 min.
But One Finger Death Punch is actually more than that. It's one simple idea, that has been polished, extended and implemented with infinite care, until the devs created a fantastic gameplay experience.
First of all, instead of a simple "one size fits all" approach, the game adapts to you : the better the play, the faster it goes, if you struggle, the game will slow down. That will help you learn the game at the beginning, but also will keep it challenging all the time, as it will get harder as you get better. That's how the game manages to keep "in the zone", in that transe-like pleasureable state where you seem to be able to defeat all problems.
Second, to keep it interesting for the long term, the game tries to vary the level goals and gameplays. Even though you're still only using 2 buttons, the devs found many ways to make it feel different : fast levels with a very good weapons, levels with reduced visibility, bosses levels, etc...Moreover, you can also select your level on the huge map, which allow you to avoid the stuff you don't like, and focus on the things you do like.
At every step of the way, you can see that the devs polished the experience to make it as good as possible, for as many people as possible. Players who just want to relax will like to get in the zone and enjoy the cartoon violence, while people looking for challenges will have tons of hidden stuff to find, highscores to beat, and crazy new challenges to discover.