Scratch player here, maybe 250 carry off the tee, 7-iron 165ish, medium trajectory.
Hard to me:
1. Many holes with long forced carries on the approach. I do not mind this every now and then for variety, but my confidence goes down when I'm more than 180 yards from the hole with no bailout.
2. Greens that are fast for fast's sake. My home course has very good fast, smooth greens, but they can be borderline unplayable on tournament days. I've played a handful of green sets that are faster (I'm talking more than 11), and its no fun at all when even a 10-footer can turn into a calamity, or you can't go for a putt of any length all day.
3. Pinehurst-like fairway chipping areas where you must chip/uphill, sometimes a decent distance. I have a good short game, but the 50-50 decision of whether to pitch or bump drives me nuts. Ultimately I could make both work, but I would still love the choice to be simple each time.
Interesting comments Tom on the UK/USA perception of difficulty. During my first trip to Scotland in 1999, the dominant difficult feature of links golf to me was fescue lining most holes. I came away thinking that if I grew up in Scotland, I'd be the straightest driver of the ball ever, because you have to do so to survive on those courses! I don't buy the "width creates too many choices" difficulty argument...I've always thought American parkland courses need to be wider, and the fairway widths seem arbitrary, mowed well in from the treelines to meet a "goal width." I loved the fairway presentation at Old Elm and Shoreacres (August 2018), where you have enough width for even amateurs to pick a side, and only something 20-30y offline found the rough.