First, it throws you into the deep end with very little in the way of explanation. My pre-owned copy came without a manual -- don't know if one was supposed to be included or not. (I don't normally even bother to look at the manuals of most games because these days they all have comprehensive in-game tutorials.) Dark Souls is a hardcore RPG that has you poring over stats, weapon effects, etc. etc., and much of it is barely explained. The good news is that pretty much anything you need to know can be found on the Internet.
Second, if you don't tread carefully you will be punished. The very first enemies in the game can kill you without too much trouble. Close-quarters combat is deliberate (I haven't tried ranged combat yet). You have to time your weapon swings and keep your shield up -- if you mis-time your moves you'll leave yourself open and you can get taken down with a few hits. You have to explore new areas slowly with your shield up because enemies can jump out at any moment. It's pretty nerve-wracking to be honest, but that's mostly because...
Third, the save system works like this: periodically you'll find bonfires which you can rest at, and restore your health and stamina (among other things). When you die, you return to the last bonfire you rested at. However, if you rest, or if you die, the game kind of "resets" and all the enemies you killed are returned to life (except bosses -- they stay dead). So if you've just spent an hour fighting your way through a horde of enemies and you weren't able to find a bonfire before you died, you have to do it all over again. And again... and again. (Fortunately, items that you found before you died will remain in your possession.)
Fourth, when you die you lose all the Souls you were carrying. You can do useful things with Souls, like buy items or level up, and you get them by killing enemies. Here's the thing though: if you can fight your way back from the bonfire to the spot where you died, you can pick up all the Souls you dropped. However, if you're killed again before you get back to the spot, you lose the Souls permanently.
Finally, I've barely gotten any distance into the game and I've already encountered two big boss-like creatures. The first one I managed to kill after a couple of tries but the second one has kicked my ass every time...
***** SPOILERS *****
I'd just spent a good bit of time slowly picking my way through castle ruins, fighting Hollow Warriors -- zombie-like soldiers, which can be killed fairly easily as long as you're patient. I'd probably killed about a dozen of these things before coming to a very long, wide rampart. There were two Hollow Warriors on the tower behind me so I climbed up and killed them, but by this time I'd run out of Estus flasks (which restore health). So I climbed down the tower and proceeded very slowly along the rampart towards the tower in the distance, shield up, hoping I'm going to find a bonfire before I have to fight too many more enemies.
The next thing I know, a huge demon jumps off the tower in front of me and comes charging down the rampart. I crap myself and run away, and it kills me with one swing of its giant axe. Zoom, all the way back to the last bonfire. Yep, now I have to fight all those Hollow Warriors again, just to get back to the demon that killed me with one hit.

I did try it again a couple more times -- fruitlessly -- before deciding I needed to level up a bit and buy some better weapons and armor. So I spent my last play session just hanging around the bonfire killing Hollow Warriors. Tomorrow I will have another go at the Taurus Demon!