Zeboyd's Robert Boyd, who you might remember for working on retro-JRPG-like titles such as Breath of Death VII and Cthulhu Saves the World, has written a feature about Dark Souls' design, and specifically the ways its perceived difficulty is made to appear higher than its actual difficulty. Here's a sampling:
5. A deadly non-linear world... except, not really
Non-linear worlds are inherently more difficult than strictly linear worlds because the player is less likely to know what they're supposed to be doing and is more likely to run into areas that they're unprepared for.
Dark Souls appears to be very non-linear at first glance, but in actuality it's a lot more linear than it seems. To start, individual areas tend to be very linear -- albeit with hidden treasures to be found in various side paths. But as far as the game's overall progression goes, Dark Souls has a heavy reliance on gating. The game opens up a portion of the game to the player and then to access the next set of areas, a certain task or series of tasks must be undertaken.
Excluding the tutorial area, there are basically three major goals that must be accomplished in order to reach the final boss. The first goal is to ring two bells (each of which is guarded by a boss). Starting in Firelink Shrine (the initial hub area), there are three areas the player can travel to -- The Catacombs, New Londo Ruins, and Undead Burg.
Powerful skeletons that come back to life soon after being killed guard The Catacombs, and New Londo Ruins is filled with ghosts that are invincible unless the player is cursed, or uses a certain item. In contrast, Undead Burg has enemies that are similar to the enemies in the tutorial area and is the obvious choice to start out with. Right from the start, Dark Souls is subtly funneling the character into the course of least resistance, and it continues to funnel the player through the entire two bells portion of the game, by giving the player keys that indicate where they should go next.
After the two bells have been rung, the second part of the game begins with a cutscene that shows the player that a huge fortress that was previously locked has now been opened. The player's course is clear -- explore the fortress and the area beyond it. This portion of the game is one of the most linear, with two areas that must be completed in succession and only one optional area. This section acts as an exam: If the player can completes these two areas (some of the hardest in the game so far), they're deemed worthy for the third main portion of the game, where the entire world (minus the final boss area) is opened up to them.
By gating the areas of the game in this manner, it allows Dark Souls to have a more measured difficulty curve than a truly non-linear game would allow. The areas opened up in the third part of the game tend to be more difficult than the areas in the second part of the game -- which, in turn, are more difficult than the areas in the first part of the game. This helps to prevent the player from getting truly confused and lost like they might if the entire world was accessible right from the beginning.
7. Combat is a replenishable resource
One of the smartest changes Dark Souls made over its predecessor was the switch to the bonfire system. The previous game, Demon's Souls, uses a traditional resource system where the player can restore their health and magic points (MP) with items that they can find and purchase. However, in Dark Souls, the player is given a set number of heal potions to use. Additional potions cannot be found; however, the player's potions are restored every time the player rests at a bonfire. Likewise, the player is given a set number of spells they can use each time they rest at a bonfire.
The replenishing resources bonfire system has a number of advantages over the way that Demon's Souls did things. It encourages the player to use all of their magic arsenal instead of just the spells with the greatest return-on-MP investment. It prevents the player from stockpiling huge quantities of health and MP items, thus rendering the resource system largely irrelevant. And it removes the need to grind out money and item drops when you're low on potions. Through the bonfire system, the player is encouraged to use all of the resources at their disposal, since they know they'll recover them next time they rest rather than having to worry about hording resources.
While I agree that the game's "difficult" reputation might be exaggerated and don't really disagree with any of the points mentioned, I'd argue that a big part of why the game has this reputation is due to the fact that, while none of the challenges presented is anywhere close to impossible, the game punishes mistakes rather harshly, especially when put side to side with most comparable modern titles.