Dark Souls is a great game. Not because it's hard, which it is on your first and second playthroughs, but because they made the game a learning experience. You can take your experience (not the number kind) from character to character, and do exactly what you did before and get (mostly) the same results. The game is fair. Asshole-ish in some parts (Capra Demon) but fair. It always presents you with the same scenario as before. The only inconstant variables to the game are you, the players. Your set up could be different, other players can invade you, the enemies might jump left instead of right, but the core of the game always stays the same. The way they handled NG+ was also damned good.
The story, how they sort of hid the story in plain sight was just amazing to me. Everywhere and everything has it's own story, from Sunbro Solaire (my picture) to the boss in Lost Izalith, but nothing comes straight out and says "Here's my back story. Now that you know, let's fight." Hell, the game doesn't even give you directions on where to go in the beginning, other than "ring the bells," and even that is forced out of the second non-hostile NPC you find. The game is just the perfect amount of exploration and action for me that I keep going back to play it again.
EDIT: There's also a couple of silly hats in the game.