Patrick,
I think two things that come immediately to mind for me are playability and fun, and they are very interrelated.
Most of the courses that will get mentioned here are ones where the mid to higher handicapper has a fighting chance, and while their errant shots end up having a negative outcome, it is not often at the cost of a lost ball and stroke.
I played a course yesterday that would fit nicely into the recent "penal" course discussion, and for the average golfer, it's probably a 3-sleeve course given the constant narrowness, everpresent wetland hazards, and any number of forced carries. I couldn't imagine going back to the first tee at the end of the round as I was physically exhausted, even if mentally untaxed.
Instead, I love a course where I can stand over any shot and consider multiple ways to play it, depending on conditions. To me, that's the FUN factor.
Courses that dictate only one type of shot too often become stultifyingly dull, even if "challenging" in the conventional sense. There is simply no thought involved, and it all becomes about rote execution. On the other hand, if I feel that I'm in control of what I can attempt, and the architecture facilitates that freedom, then I may pay a price for my choices, but at least they're mine, and I can try something else the next time.