I was reading a debate over whether Dark Souls is difficult or just different when I realized it was similar to my difficulties playing The Banner Saga: most of the difficulty came from wrapping my head around game concepts that felt contrary to years of tradition. Familiarity with other games in the same genre can be detrimental when dealing with such games, since you're fighting years of habit to play the game properly. Here's a list of games that have this type of difficulty, and my explanations for why they do:

Dark Souls: Probably the best example on this list. Dark Souls is notorious for being utterly difficult, but people who have played it swear it isn't that hard. The key is learning to slow down and not be impatient. Dark Souls teaches players to adopt a slower pace (and reverse years of rushing through action games) by brutally punishing them whenever they get reckless. Peeking around corners & waiting for a solid opening to attack is recommended, instead of charging blindly forth and wailing on enemies.

The Banner Saga: Focusing down one enemy at a time leads to defeat instead of victory here, thanks to the interplay between 2 mechanics:

1. The strength of characters' attacks are tied to their health. A character near death will barely scratch its enemy.
2. Turns always alternate between the two sides, no matter how many characters are on each side. (A side with 4 characters gets the same number of actions as a side with 12 characters.)

An enemy with 3 healthy, strong opponents will wreck your champions, while an enemy with 9 injured opponents will be quickly mopped up by them. I had to break my old habit of killing each enemy before moving on to the next one, encouraged by dozens of games where an injured enemy hits just as hard as a healthy one.

Counter-Strike: It's an old game, but I can imagine how much FPS players struggled with this game when it first came out compared to Doom, Quake, and Unreal. It had no respawning until the match was over, no weapon pickups, and attacks were more lethal. Like Dark Souls, it discouraged charging in recklessly by making death painful.

Other games that would be difficult due to being different: Genre-creators & any game that changed its mechanics in a way where previous experience hampers players. I believe Dune 2 (created RTS genre), Warhammer 40k: Dawn of War (squads, morale, gain resources by taking objectives), Ikaruga (you want to be hit by certain-colored bullets), and Disgaea (geo panels, stacking/throwing, purposely "broken" leveling) are other examples of this type of game.

Games that are not difficult due to being different: Games like Super Meat Boy, Spelunky, and I Wanna Be The Guy are difficult, but they aren't that different from their predecessors. Gaming veterans can quickly adapt to their mechanics, rather than being tripped up trying to play counter to years worth of habits.

What other games do you think are difficult due mainly to being different like this?