You will die. Hundreds of times. I know a lot of people hear that and go in expecting either it to not be difficult, or for deaths to be complete BS. The unique thing about Dark Souls, however, is that it is not.
When disaster is near, there are ALWAYS hints that let you avoid something. And the deaths are almost never the game's fault. There is not a single 'that is total bull-blam!-' moment. Every death you have, you will know that it was completely your fault. That's really rare in a game, and make it INCREDIBLY satisfying to finally beat a part you have been stuck on... for a month... -blam!- you Seath the Scaleless you can suck my balls.
A single playthrough will net you 50-70 hours of gameplay. More if you're a hoarder and want to collect everything. Then there are infinite new game plus modes. The first NG+ is a substantial increase in everything, the second one less so but still fairly big, then the rest are very small increases in difficulty/ experience. Very small.
The story is almost nonexistent to the casual gamer (and a casual gamer playing this game will NOT have a good time- this is a much more hardcore game). However, if you delve into things such as item descriptions, you will find an incredibly substantial and deep lore that is very interesting. A lot is left up to fan interpretation. Personally, I love listening to these videos, as they really go in depth into the lore and catch things I never would have caught on my own. It's fascinating.
Base line? The game is fun. Pure, simple fun. The game is insanely difficult. You will die, but the game is about learning from each death so you know what NOT to do next time. There is always a strategy that will make every situation easy. The graphics aren't too special, but the art style is obscenely beautiful. Story is hidden, but expansive if you take the time to search for it. You have to search for it because absolutely NOTHING is handed to you. You won't know how anything works aside from a few basic moves- you have to figure out everything yourself or from vague references.
Don't google or youtube anything. Except 'ceaseless discharge' when you get there and something called 'the abyss.' I'd google those when you get to those points, because they're hard if you can't figure it out. And the Abyss in particular is really vague. But yeah. Otherwise? It deserved GOTY.