I'd argue that is when a game developer has failed. One should not be required to make their own restrictions to make a game challenging. It is their job to entertain us, not for us to entertain ourselves.
I agree and disagree with this statement. Yes, if you are forced to handicap yourself on your first playthrough to make it fun then the game developer has failed. It is complete BS for people to say "maybe you just shouldn't have done 'x' and you would have had more fun."

However I don't see how this can apply to future playthroughs. Once you have already mastered the game and beaten it one or more times the difficulty curve will obviously be completely different for you than a first time player. So you might want to spice it up a little.