I'm talking about motivation, when I have to repeat an area I have beaten before (especially if it's really not difficult to do so) I get bored because, well, I'm redoing stuff for no reason. What reason is there for me to redo the whole game just for one interesting part that's at least 10 minutes in? That's wasting more time than MMORPGs. I can handle repetition a few times but with difficult games you'll do it over and over again.
Ninja Five-O wasn't too short. Well, yes, it was since you don't have enough ninja action after finishing it but that's more because it's good rather than quick to finish. An area that takes 10 tries with restarting from the beginning is more annoying than an area that takes 50 tries with each starting right at the area. Of course I would have put up with starting over back then but that's because game design has improved since back then. As I said, I don't mind difficulty but I mind unnecessary punishments for the player because those kill my motivation dead. Try playing Bunny Must Die, the bosses are really F###ING hard but you get a savepoint before them so you can focus on mastering the fight itself rather than replaying areas you can already beat blindfolded.