Sell me on this game.
Even though it is hard as fuck, it is also very rewarding. There is nothing more satisfying than to spend hours trying to perfect how to get through a level and then killing a boss. Even if it takes numerous attempts, it is really fucking satisfying.
Also, you can randomly jump into another player's game and kill them.
Also, if another player jumps into your game, you can kill them.
One time, a Black Phantom (another player) invaded my game in world 4-2. I was hidden in a small cave. The Phantom runs in, but I have a ring of limited invisibility. He gets close to me and I let loose with a one-shot kill of inferno-hell-destruction!
It was fucking beautiful.