Dark Souls will primarily be remembered for its considerable difficulty. It's an absolutely true shame, of course. To my recollection, no other game's punishment caused my brother to literally break a controller in a fit of rage, delete his save and write a chart-topping tribute song before his hatred could subside.
It's not all death and discouragement, however. Though every twist and turn of Dark Souls' menacing and lovely locales plays host to a preset lineup of fiends (many of whom can kill you with a sideways glance), there is a strange progress to the proceedings. It may come after hours of un-progress, but it comes -- and when it does, it comes correct.
Self-improvement in games typically comes in forms that are mechanical (you level up and gain five magic points!) or educational (now you know where the spike traps are). Dark Souls' core tenet of repetitious short-burst failure paired with a wildly open-ended RPG character progression system blends both methods perfectly. What's more, it offers an intensely terrifying risk/reward system for ever-precious Souls; fortunately, your Brains are never jeopardized.