8. Exploration is a limitless resource
Zelda has bombable walls. Dark Souls has fake walls. The difference? In the Zelda games, you can only test walls as long as you still have bombs left in your inventory -- but in Dark Souls, any kind of attack (even just a harmless roll) is enough to test if there's a hidden passage behind a seemingly solid wall. The player is thus encouraged to search for the game's many secrets -- because there is no penalty if they guess wrong.
9. Their bark is bigger than their bite
An easy way to make a game feel harder is through appearances. A great example of this is the classic horror game, Silent Hill 2. Mechanically speaking, Silent Hill 2 is a pretty easy game, but by making the monsters and locales in the game look horrendous, the game feels a lot harder than it actually is.
Dark Souls enemies are almost universally grotesque and are frequently much bigger than the player character. But Dark Souls takes it even further with many of its early bosses appearing above the player before crashing down in front of them. The implication is clear -- the player is insignificant to these monsters and they won't hesitate to squash him or her like a bug.
However, the experienced player will soon realize that many of the scariest-looking bosses in Dark Souls are also some of the easiest bosses, with easy-to-read tells and attacks that are easy to dodge and counterattack. By making the bosses and monsters look intimidating, their perceived difficulty is increased even if their actual difficulty isn't.
10. Combat doesn't require fast reflexes
Most difficult games require lightning-fast reflexes. Not Dark Souls. Combat in Dark Souls is a methodical affair: Block then attack. Dodge then attack. Attacks are slow, and easily punished if missed. Even drinking a health potion takes several seconds (unlike most action-RPGs, where it's instant). The game rewards the players that can keep their wits about them and actively punishes mindless button mashers.
11. The stakes are high... for both sides
Yes, enemies in Dark Souls can deal massive damage to the player. But you know what? The smart player can deal massive damage to the enemies as well. From massive swords that can kill a group of enemies with a single strike to arcane spells that can decimate bosses from a distance in next to no time, there are many ways the player can become the scariest thing in the entire Dark Souls world.
One of my favorite "tricks" to breaking the game early on is through the use of the 2-handed button. Equip the easily obtainable hand axe (a few other early game weapons will work as well, but the hand axe is the best for this strategy). When you see an enemy with a shield, switch to the two-handed stance and then just go to town on them. The hand axe has a high stagger stat so, in no time, your assault will knock the enemy's shield out of the way, allowing you to defeat them easily. In a typical game, enemy shields would be impenetrable, but in Dark Souls, the enemies play by the same rules that you do (at least, until the enemies start to dwarf you in size).
Observant players can find even more tricks to break the game, like powerful weapons hidden just off the main path, and shortcuts that allow access to areas long before they are traditionally reached. Dark Souls rewards the player who isn't afraid to fully explore and then break the rules.
12. Death is punished... some of the time
At one end of the death/punishment spectrum, you have your typical roguelike with perma-death, where a single death means you need to start the entire game over. At the other end of the spectrum, you have your average game these days, where death sends you back to a recent checkpoint.
Dark Souls finds a happy medium between the two extremes. If the player is killed twice in succession, they lose their current souls and humanity, an important (but renewable) status. But if they can successfully return to the place where they died last time, they don't lose a thing. Also, both souls and humanity can also be obtained via certain items that are not lost upon death, allowing the player to safely save them up for when they need them (like for the next new merchant). The player also spawns from the most recently used bonfire, meaning that even physical progress isn't necessarily lost.
Dark Souls is a difficult game; there's no question about that. However, by making the game appear even more difficult than it actually is and then by subtly lowering the difficulty in a variety of ways, the developers have made the difficulty in Dark Souls manageable -- and fair. And when victory is achieved, it is made all the more sweet for appearing to be more difficult than it actually was.