Some games aer fun w/o being hard. Once I had the invisability code for Manhunt, it was just more fun. Crackdown stops being fun on harder difficulty. Fallout is only good if you can explore. Silent Hill... well, that's hard even on easy mode (First one).
Left 4 Dead, on the other hand, gets more challenging because you are running a gauntlet. You are trying to get to the end without dying with at least one survivor. That's fun especially when you shut the door on a teammate while umpteen million zombies flood into the room and tear your buddy apart.
Cheat Codes make games easier and sometimes more fun, and they are becoming more and more rare. Unlockables aren't common anymore. And bonus content relys on your ability to beat SUPER EXTREME HARD MODE! two or three times. And I don't know about everyone else, but some games have a Bullshit level or something near the middle that is almost impossible to pass no matter what.