Here is my breakdown using Ballyhack as the reference. I think of challenging as shots that ask a question that I have the skill to execute if I hit a good shot and usually include a bail out that I can play my miss to if I don't take the challenge on too directly. Difficult on the other hand is a shot that I know I can pull off if everything works out but I know that my dispersion pattern makes the eventual outcome have more chance than skill, also these usually do not have a bail out to work the likely miss towards.
Difficult shots at Ballyhack given these definitions and my skill set:
Tee shot on 3rd hole: As Tommy said - severe green that leaves likely bogies from even decently struck near misses if you end up releasing down the wrong slopes - from the back few tees this is a very difficult hole
Tee shot on number 12: Bunkers right and left - difficult shot that requires precision with no side to bail out to - less demanding than it was at one time when there was ball eating gunch on both sides of the fairway
Approach on 16: Depending on the tee shot and tees played I seem to have between 140-200. Green is angled away from left to right with no good misses other than fairway short of the green. More forgiving since the rough uphill from the green on the left and long is now more playable.
To counter this with some shots that I view as especially challenging and why:
Approach to 2: Green slopes to back left and shots tend not to hold well (at least mine tend not to) - Depending on where you left your second and where the hole is relative to the lion's mouth bunker there are many ways to play the shot given the usual fast and firm conditions. Generally you can always chip or pitch something that will put you on the green but not necessarily close.
Approach to 4: The three distinct areas of the green (front left - back left bowl and upper right tier) and the difficulty two putting across the transitions this shot is very demanding with a lot of decisions as to how to flight the ball in given the slope of where you left your drive. If pin is front left there is plenty of room to lay up and leave a decent chip - back right you can play long and leave a decent chip or run one back there right to left across the green - right shelf is the most difficult but you can bail into the lower part of the green and try to two putt up the hill.
Approach to 18: All uphill to a huge green. Figuring out the right club and how you want to try to flight the ball up the hill and usually under or around the big tree is always interesting. Large space to hit into, but tough to get on the correct part of the green unless the hole is in the swale.
I could have named many more challenging shots on the course - in my mind they involve a decision as to how precise I want to attempt to be with a side or distance that I can hedge to either through shot shape or club selection. Difficult on the other hand is a shot that asks for an answer with no real hedge available, you either execute the shot or deal with the consequence without the option of a hedge.
Tommy - I would love to hear your comments on this as it relates to Ballyhack. A course that I know we are both familiar with and one that certainly has different shots that fall across the spectrum that is being discussed here. BTW I'll be down there with some guys 5/9-11 if you happen to be around.
Good question. I've thought about it a lot. The first hole is challenging but maybe not difficult. The tee shot looks scary because of the forced carry but the fairway is miles wide and the carry isn't as long as it appears, except from the way back tees. The it is terrifying. The second shot is to a difficult green and is uphill but less than 150 yards. It is challenging to hit it near the pin. The third hole is just plain difficult. The green is among the most undulating and the course. As I look at the entire course it is full of challenging shots but overall it is just plain difficult. The fun of the course is to try to decide what the hole requires then try to figure out what your particular set of skills will allow.