Phil:
Yes, the best holes are usually hard holes.
But that does not mean the converse is also true, that the hardest holes are usually the best (or even good) holes.
Lots of people do take the latter as gospel -- ask a club professional to name the best hole on his course, and probably 75% of the time he will name the longest and toughest par-4, even if it is as straightforward and banal as could be. Most members will follow suit.
But, to go back to your original question, you can't have a great hole unless there is a sense of accomplishment to be gained from it, and an easy hole leaves less sense of accomplishment. Dr. MacKenzie was perhaps the first to note that for many players the hazards are there to give psychological thrills when you carry them, as much as for penalizing the missed shot.
As to your question about "half-par" holes, I don't know the answer, because most easy holes are labeled as half-par holes. The best example I can think of at the moment is the 13th at North Berwick, the Pit. Practically no one ever tries to drive the green, so it is not a half-par hole by that definition; but it is a fairly short par 4 and there are just two tiny fairway bunkers and one defense for the green, the wall. I've made birdie more than once in limited plays, so it can't be that hard. But I believe it is a great and strategically interesting hole nevertheless.