Challenging, enigmatic, immersive, and incredibly fun, Dark Souls improves upon it's predecessor (2009's PS3 exclusive Demon's Souls) in almost every way. Players traverse the open world of Lordran, from the top of the castle bell tower to the very bowels of hell, engaged in incredibly nuanced, smart combat with varied, hostile enemies. As the tagline says, you will die, and the game uses death not as a punishment or a falsified 'end' condition (vestigial design from quarter-eating arcade games), but instead as a learning tool. Death is also rarely unfair, being a result of your failure, not bad game design.
If you're looking for something easy and relaxing that will constantly pat your head and tell you how awesome you are, Dark Souls isn't it. It's a tough nut to crack. Players are given the barest amount of information to proceed and a big part of the fun is in discovering new areas, experimenting with different play-styles and figuring out what it all means. The online functionality - more-or-less the same 'massively singleplayer' approach that Demon's Souls had - remains as the single most interest example of non-traditional connected play in the history of gaming.
I don't think of Dark Souls just as a great game. I also think it represents brave, intelligent and ground-breaking game design - something very rare in Shooter Season 2011. If you've got the gumption, there's nothing else like it.