I have never really understood the (apparently accepted) wisdom that difficult cannot be fun. Two of the most enjoyable (and memorable) rounds of my life have been on courses generally accepted as very difficult (Carnoustie and Wolf Run) in rounds where I have struggled to score even moderately.
Sean touched upon the importance of recovery shots and the availability of those (even if very difficult) helps make difficult fun. But also the ability to get your ball round. Difficult shots that reward being played well but leave a difficult shot if you fail to execute are an important part of the recipe, I think. Difficult shots that if you fail to execute you have to play again, or take a drop, or spend time looking for a ball you're just going to have to hack out are not fun.
I don't like easy (and that's one reason I rate Kingsbarns much lower than most). I do love half par holes (one reason I love Elie so much), and I guess I don't mind a course with a load of half par holes where they are half over, rather than under or a mix, and I don't mind holes where I'm going to be really happy to play well enough to make bogey, if the path to that bogey requires me to execute some challenging, fun, shots.