Just ignore the Ward vs. Shivas battles about Bethpage versus Medinah. Some people confuse difficulty with challenge...
Its trivial to design a difficult course. You or I or anyone could design the most difficult course in the world that'd make Koo'lau look like your average 9 holer muni. John Cleese has some good ideas in this regard, along with Dr. Evil -- fill the water surrounding an island green with sharks with FRICKIN laser beams on their head and even Tiger will be sweating it when he arrives on the teebox! It wouldn't be something anyone would want to play, of course.
But if you make something that's challenging, that's another matter. It can be as difficult as you want, as long as there are better results for more skill (in shotmaking, thinking, etc.) and there are options for all (or at least a good portion of) levels of golfer that are within range of the ability they'll bring on a good day. As I see it, the art in architecture comes in finding that balance so that all golfers are challenged to produce the best they are capable of, and punished for slacking on that aim, without requiring them to produce something they can only do once in a blue moon or through blind luck.
Probably the hardest task for a designer is to design a course that isn't particularly difficult, that isn't going to punish every little mistake, at least not in an obvious or overt fashion, but is still widely regarded as fun and challenging even for accomplished golfers. Look at courses like North Berwick, highly ranked and regarded, but not particularly difficult and certainly not very penal. This type of course has even been making a comeback lately...the pendulum has swung back from some of the excesses of the Pete Dye style.
Personally, I prefer a course that will beat me up a bit when I make a mistake. But that isn't for everyone. Some people would rather have every hole be a birdie hole. Those people probably like offensive battles in football, while I like a good defensive battle.
Even for a course that'll beat you up, there are different ways to do it. I'm not a big fan of a course that will pretty much show you where you need to be by virtue of bad stuff everyone else, and then throw penalty strokes or chip/wedge outs for a poorly played stroke. I much prefer a course that will let you BELIEVE you are getting away with something on a missed shot, then come back to bite you on your next shot or the one after unless you play a truly superior shot to make up for it, forcing you to think your way around the course and being more subtle by punishing you for mental mistakes. I bring out my best when I mess up, and have to do something spectacular to get back on track. And while I don't succeed every time, it is certainly more satisfying when I do. Guess its the old "I'd rather make a par from the parking lot than the middle of the fairway" thing.