In my humble opinion game difficulty is a balance, different for each title. In the afore mentioned piece, Kirk Hamilton claimed that Assassin's Creed's combat was too easy, and that second playthroughs of the game would benefit from increased difficulty in that area. I'm not so sure - the appeal of Assassin's Creed, for me, is the change of pace. It's a game I can saunter through, a game I can casually explore. Red Dead Redemption evokes similar vibes for me.
There are games I think should be difficult - Dark Souls for example, a game that depends on learning techniques and applying them. Metal Gear Solid, for me, falls into this category.
But that's enough from me - what do you guys think?