.....When designing a golf hole, at what stage does it become to difficult or unplayable for the average golfer?
.....In my experience, generally not until opening day!
(insert rim shot here)
But seriously, though.....good question.
I am always amazed at where high handicap (and who am I kidding, even my shots) often end up. For total unplayability, there are some rules of thumb -
Minimal forced carries, with none over 150 yards from senior tee and 90 yards from the forward tee. Minimum forced carries to greens.
Turf areas for most holes about 225 feet wide (about 75 yard minimum, with 83 or more preferred)
Fairways that hold balls - ie no excessive cross slopes over perhaps 5% (maybe up to 9%, depending on turf)
Greens with cup areas at 2.25% or less)
Minimal blind shots, and especially blind water hazards.
Sand bunkers you can't see out of are tough, as are bunkers placed on the front right of greens or other high play areas.
Too many long holes, like over 600/500/250 yard par 5/4/3 holes, even with forward tees. They cause an extra shot per hole which is no fun.
As to EXACTLY when it becomes too tough, that is hard to tell. Obviously nothing wrong with an occaisional force carry, narrow fw, contoured green, etc. Wind of course, plays a role. However, at some point, if the designer breaks too many of these rules on too many holes, it gets too hard.
I guess most of us would know it when we see it, but perhaps some others could contribute some other "excessively used features" that would tip the scales the wrong way.