Now that this is talking about game design:
The difficulty of a game is irrelevant. It's the fun-ness of it/the connection the player has with the game. Games like I Want To Be The Flash game fail to entertain many users because it puts players through the same tasks over and over again until they FINALLY manage to succeed all of them. In my opinion, a really fun game is one that only gives achievement, but no harsh punishment. The goals are laid out for the player, and it is made clear what is expected of him, but he should always have the feeling of advancing and discovering something new. Games like Bioshock sort of solve the repetition problem by not resetting the world when the player respawns, so all his previous achievements remain, and he is able to move onto the next task.
Overall, the game can be very very easy, or very very hard. It's all about how you present your ideas, story, etc., and above all, contrast between all parts of the game. Like in music, there's a fine line between repetitive and geniously thought out, progressive.
That being said, there are always exceptions. You can't define fun, better have some beta testers take a whack at it!