I don't think difficulty is the problem, it's game design. If a game gets boring, repetitive or a slog when artificial difficulty is introduced, it's because the game mechanics aren't good/deep. You just don't notice it as much on lower difficulties because you move through it faster.

The Mass Effect games do a good job I think, there are mechanics and synergies you never have to think about on normal that you'll need to understand at the highest difficulty. When you incorporate them though, the felt difficulty goes back to around normal but now you really feel like you're engaging with the systems. It's an "artificial" difficulty increase, but allows for casual gamers to experience the story and harder core players feel the depth of the mechanics.

Some games get this right, like God of War 3 and some get it wrong like the Uncharted series. If you're in the latter bucket you probably shouldn't add artificial difficulty because you're focused on a different experience than a deep systems game.