Video games are a unique animal. You can't change a movie's rating on the fly to suit your tastes. A game of chess won't get any easier unless you pick a chump to play. And you can't re-write Eminem songs to add extra profanity. But video games put those decisions in your corner - Choose Your Difficulty - and many force the issue right up front, before you're allowed to even start playing. If you're suddenly wondering why that is, the answer's pretty simple. A smart developer knows their game's dead in the water if they don't nail the difficulty.
Forget graphics, forget weapons and puzzles, forget story, forget characters, and forget controls. Difficulty is the single most important element of any game.