Dark Souls has some interesting ideas in terms of world design, and is a rather immersive game. This however, gets buried in a game that ultimately feels rather shallow, overly monotonous, and unrewarding.
Let's start with the positives:
The game-world is unique with its interconnectedness, it's not something you see in most action-RPGs. You can do different zones in different order, although some do require you to have completed a specific one beforehand. Still, it's very impressive how it's all connected. It's a clever design.
Another strong positive is how creative they have been in making each zone in the game, the different types of enemies and structures. It is creative and playful, and takes me back to my childhood of drawing platform maps for some imaginary game. This is the best thing about Dark Souls in my opinion. Granted, you get all the generic Fantasy stuff of skeletons, dragons, goblins, old castles and such, but you also get an equal number truly unique enemy-types and architectures. For example, the hidden mushroom zone and the mimic chests.
The boss designs felt quite unique for the most part. Each boss requires a somewhat different strategy, not in terms of combat mechanics, but in terms of positioning; in using the combat arena to your advantage.
The grainy graphical style looks really good, and is perfectly fitting for the game's dark and otherworldly atmosphere. The sound design and soundtrack were both really good; the former created an eerie atmosphere in the game, the latter made the boss fights feel epic and glorious.
So Dark Souls has plenty of positive attributes.
For the negatives:
The gameplay was fun at first, but after the 20-hour mark it started to feel increasingly reptitive and tedious. I feel the game is far too long for what it is. There's little variation in the combat, which gets enhanced by the gameplay being very slow. You swing your weapon, roll away, then swing again. Over and over and over. The weapons don't bring enough variation since the game does not encourge using a wide arsenal, unless you are willing to spent dozens of dozens of hours grinding for resources. I think Demon's Souls was much better in this regard, by being a much shorter game at just around 20 hours or so.
The other major issue I had concerns the storyline and narrative. One of the many functions a good narrative serves is to encourage and motivate the gamer, it tickles your imagination with its lore and makes you want to continue playing the game even when things start to feel tedious. Dark Souls doesn't really have a narrative, let alone a cohesive one, you only get tidbits that seem randomly distributed across the game-world. The sparseness of it makes the gameworld feel empty and shallow. There are very few NPCs around, to add more story content and narrative. On the outside Dark Souls looks rich and deep, but when you are actually in the game it's anything but this. Put differently, for a 60+ hour action-RPG, the little storyline content and narrative the game has is stretched far too thinly.
A good storyline and narrative gives also purpose to your actions. Even after 30 hours or so in Dark Souls, I still had no grasp of what the point was in terms of storyline. You simply go from one zone to another slaughtering enemies for the simple reason of them 'being there'. For anything RPG-related, this is very bad. The game started to feel rather shallow and pointless at this stage. 'Why am I here and what am I doing all this for?' I asked myself whilst playing the game, .... but the game had no answers to this.
The developers said in an interview that all this was intended, that gamers were supposed to discuss and interpret the storyline in a communal way. There are many flaws with this idea. For one thing, it only works if you take pen and paper and literally write down every piece of lore you come across, and then you compare notes with other people. That does not sound like fun to me. Nor is it rewarding that all you get from the game, as far as storyline goes, is simply different people's interpretations of it. The very few people who could be bothered writing everything down that is. It gets even worse though, the developers said that what the NPCs say or write may not even be the truth, that they could in fact be lying. This utterly destroys any credibility the storyline and narrative has, you might as well invent your very own storyline at this point.
Being able to make character choices is a core element of RPGs, yet you only get a handful of these moments in Dark Souls. As mentioned, there are very few NPCs around, and you rarely get to make a choice that affects the storyline or character relations. So I don't consider Dark Souls to be an RPG, especially one that is comparable to games like Skyrim, Mass Effect, or Witcher.
Not having more save-points or manual saves simply made the game feel more tedious to me. It did not add much to the challenge as you could simply sprint to the boss in the majority of cases, that's not what I would call 'quality gaming' or a good use of my game time. It simply becomes annoying.
The production quality is not that good, as the game is quite glitchy. For example, your character will swing through metal bars with his sword and can hit through closed doors, likewise the enemy can swing through a pillar of stone. The hitboxes too felt unreliable, especially during boss-fights. Add some terrible camera angles to this and you get a game that does not feel particularily well-made at the core, but instead feels very clumsy to handle and play.
To conclude: Dark Souls is a game you buy for its unique gameworld and its immersive atmosphere. If storyline matters to you, with deep and and well-made characters, and you if enjoy variety in your combat, Dark Souls is not the game for you.