I'm sick of everyone praising Dark Souls. So many people talk about how Dark Souls is the best game ever, how it's so hard but it's so much fun because it's completely fair, and you know what, that's BS. Dark Souls is full of unfair anti-player mechanics that would be called out if any other game did the same thing. I'm going to list them. Expect this to take a while. (I'm going to focus on Dark Souls 1 for this review, and I'll only discuss the gameplay, not the story aspect.)
My biggest single criticism is the amount of retraversal. Most bosses don't have a bonfire (respawn point) near the entrance, so every time you die you have to run all the way back to the arena - usually through a mess of enemies - before you're allowed to try again. Particularly bad offenders are the Taurus Demon, Bell Gargoyles, Quelaag, Ornstein & Smough (approx 2-3 minute run if you don't fight anyone - I counted), the Bed of Chaos, Great Grey Wolf Sif, Seath the Scaleless (second encounter), and Gwyn, Lord of Cinder.
On a similar note, the game doesn't return consumable items used when you die. You have a one-time use item that gives you increased damage and you've got a hard boss you need to go through? You only get one try with the item! Use it once and if you die it's still gone, forcing you to either have a very high chance of wasting the item and being forced to beat the boss without it, or else only use it once you're sure it'll be enough of an edge to guarantee your victory, meaning most likely you ended up beating the boss before you got to that point. Or the third option: just go grind to get another item. Grinding is fun gameplay, right?
The game has a horrible case of trial and error. I'll go ahead and make this a sublist:
There are a lot of enemy attacks that you can't reasonably intuit how to counter; you just have to have already seen it. It's been too long since I played for me to name specific attacks, but even many of the Dark Souls fanboys I've talked to acknowledge this (they just think it's okay).
As is typical for RPGs, the game doesn't make any effort to tell you how damage is calculated, which is important seeing as you're choosing which stats to upgrade. I could upgrade Vitality and get 30 more HP or Resistance and get 2 more Physical Defense... but how am I supposed to make a meaningful decision when I don't know what a point of Physical Defense even does? This is not challenging gameplay. It's arbitrary guessing where you're punished for not reading the developers' minds and you can't re-spec. Granted, this isn't a huge point because the consequences of poor stat choices aren't that big, but it's worth mentioning.
There are many times even outside of combat where you can be instantly killed by a threat you couldn't reasonably have anticipated. The most obvious example is the first "mimic" chest. Like most RPGs, you find chests throughout the world that contain rewards, but starting with Sen's Fortress some of the chests you find will be "mimics" which, when opened, turn into a monster and bite your head off, killing you instantly. Granted, this isn't a huge problem once you know about it, because you can attack them, but the first time you find one you have been given no clue that this sort of thing exists and you're quite far from a bonfire.
Another is the ceiling slimes in The Depths. In this area you find yourself walking through a tunnel with no foreshadowing of danger and then suddenly a ball of slime falls onto your head from the ceiling and deals massive damage, likely enough to kill you if you weren't at full health or didn't level Vitality very much. Dark Souls fans are like "Oh it's so fair! You could have looked at the ceiling and seen it!". But can you really be expected to keep an eye on the ceiling everywhere you go in Dark Souls? The game would take forever at that rate, not to mention it's the only time in the game where it would be rewarded.
Another example is the elevator in Sen's Fortress. In this area you find an elevator stained with blood. It stops briefly at a middle floor, and if you don't get off quickly enough, it goes up farther and crushes you against a spike ceiling. Again, Dark Souls fans say "Oh it's so fair! You could have deduced that from seeing the blood stains!" Except that that's not even a valid deduction. Why couldn't it just be the stain of another adventurer who got cornered on that elevator by some enemies?
There's a boss in the game (Ornstein & Smough) that consists of two enemies at once, and when you kill one, surprise! The other is magically restored to full health! And no there wasn't a way to see it coming. Dark Souls can do that and people will still defend it, despite that they'd criticize any other game for doing the exact same thing.
Finally, there's the parry mechanic. You're taught how to do this to enemies in the tutorial - basically an alternative block move that's hard to time but if successful leaves the enemy stunned and vulnerable to a highly damaging riposte. And of course, since there's no counterplay to this, it's natural to assume enemies can't do it to you. But you'd be wrong. Some enemies can do this to you and it usually kills you from full health even if you level Vitality a lot (I did). The only solution is to look at the wiki and find out about "parry stances" - apparently the enemies that can do this can only do it while in a special, recognizable stance. But there really was no way for the player to figure that out, especially since the stance is different for each enemy that can do this.
The game has fucking illusory walls. That's right, the game's display outright lies to you. The only way to be sure you haven't missed anything is to attack every wall. Doesn't that sound like so much fun?
Many of the bonfires in the game are hidden (eg. behind illusory walls), causing the experience of massively longer sections that the game is balanced for, or simply too far apart even if you know about them all. I'm thinking of the Depths, Darkroot Garden, and Sen's Fortress (god that area would be fucking miserable without the secret bonfire).
You can't fucking pause the game. That's how much this game hates you and doesn't respect that you as the player are supposed to be the center of everything here. My older brother played Dark Souls while he had a 1 year old son, and so whenever his son started crying, the game forced him to choose between losing the boss fight or leaving his baby son crying. Is that really something game designers should do to players? Also, does From Software realize they're contributing to people who argue that video games divide families because kids can't stop playing whenever?!?
Despite the Dark Souls fanbase being always on about how skill-based the game is and how you never take damage without making a mistake, there are multiple parts in the game where damage or even death is unavoidable even if you know what's coming. The Stray Demon and Gravelord Nito both require you to take fall damage to enter the arena, and Seath the Scaleless is a boss that is literally invincible the first time you fight him. When he inevitably kills you you wake up in a jail cell (from which you can easily escape) and you fight him again later in a situation where he's not invincible. Nevermind the fact that you do lose your souls when you die in Seath's first arena, it's all fair because... because...
Some bosses have long periods of invulnerability during their move pattern, while they're still able to attack you, of course. I'm thinking of the Moonlight Butterfly, which you fight from a bridge while it shoots ranged attacks at you and only occasionally comes close enough for you to hit it, and the Centipede Demon, which you fight from a small piece of land in a room where the floor is lava, and it shoots ranged attacks at you for quite a while before it finally decides to come over where you can reach it.
In an area called The Catacombs, you fight hordes of skeletons that... can't be killed? That's right, they just get back up after a few seconds whenever you kill them. You don't even get souls for putting them down. (Even the boss of the area is supported by about a dozen of them.) Dark Souls fans often praise that the enemies in the game follow the same rules as you, but this is a prime counterexample. Actually, they can be killed by using a divine weapon, but there's no way for the player to reasonably figure that out, nor is it guaranteed that the player even has a divine weapon at their disposal.
The Bed of Chaos... even Dark Souls fans usually agree that this boss is bad. My main problem with it is that it has nothing to do with the combat system you've been taught. It's pure gimmick; you just run around an arena destroying stationary nodes that die in one hit from anything and dodging sweeping arms with questionable hitboxes while the floor falls through. Combine these pinnacle-of-forget-everything-we-spent-the-whole-game-teaching-you mechanics with that the fight is actually incredibly difficult - so difficult that even Dark Souls decided to make your progress persistent, and it's still incredibly difficult - and has the longest trek to the arena in the game (exacerbated by that your attempt at the fight usually doesn't last more than 30 seconds).
I don't know if this is specific to the Xbox version of the game, but I at least occasionally experienced camera difficulties. In particular during the fight with Chaos Witch Quelaag, and just after entering the arena for Ornstein & Smough, my camera would often get stuck in the wall. Also when fighting multiple enemies at once, I found it hard to keep them all on screen, since you basically have to be locked on to fight in this game. And god help you if you press the lock on button when you aren't sufficiently in view of the enemy - your camera gets thrown in a random direction.
The weapon upgrading system is another thing I should criticize while I'm here. You can upgrade a weapon to make it deal more damage, but the problem is that there's no way to un-invest titanite you've spent on an old weapon once you find a new and better one. This creates a conflict of interest that harms the difficulty curve for all players.
Finally, the curse mechanic: some enemies inflict a status effect called curse, which fills a bar and when it fills you not only die instantly... but you come back to life with your max health halved... and to cure it, you must either use a purging stone (which there's no guarantee you have), or visit the healer in New Londo. Nevermind the fact that if you get cursed by the Basilisks in The Depths you probably don't even know where New Londo is yet. I didn't.