I should have the option of turning it down on the days when I want a "bubblegum play" experience where I can play for while and just be entertained. Then on other days, I'd like to be able to turn it up when I want to be "challenged".
Yes, there are times where I don't mind be frustrated a little, (note the emphasis!) but I don't care for situations that go way beyond that. (and I think a lot of other people do as well, judging from the recent wave of 'casual gaming' trends.)
And I absolutely hate it when I'm confronted with a dead end situation, like when I can't complete a game because some game book author made a deal with the programmers to write in special stuff that requires their game book in order to "win the game". (I won't bother mentioning which companies engaged in this behavior because they don't need any more advertising and besides, a lot you probably already know whom I'm referring to...)
So there you have it in a nutshell.