I would liken Dark Souls to essentially being Metroid with swords.
There is a heavy exploration element. You're expected to figure out where to go for yourself...it doesn't hold your hand and give quest markers or extremely detailed plot instructions.
The story is a little light. Also like Metroid Prime, you can play the game from start to finish and ignore most of the story, you'll only really get into it if you look around. But the story that is there is fantastic, once you've found it.
My greatest complaint is the controls. 99% of the time, they work perfectly...and then there's that one instance where they just fuck up royally. Now, that does happen with a lot of games, but combined with Dark Souls' difficulty level it made for some really frustrating moments. I wouldn't pass the game over from this single complaint though.
As for the difficulty level, yet, it's harsh, but it's fair, and it's actually really rewarding. It's not hard in that you need perfect technical skills in regards to playing the game, more like you just need to be extremely careful about how you go about doing things. Think ahead and tread lightly, basically. Also, be ready to play dirty: dodging so that enemies you're fighting fall off ledges is a great way to instantly kill tough enemies, for instance.
Finally, and this is just a piece of advice. When you start, you'll be given a selection of items to start the game with. Take the Master Key, which basically is a low-level key that is not consumed upon use, early on, it will unlock all kinds of shortcuts and treasure rooms.