The one thing about an Open course (and set up) that you truely can't appreciate unless you've seen it in person is how FAST the entire course plays. I've been to 2 opens to watch (Bethpage and Winged Foot) and the fairways are so firm and cut so tight that, many folks used to "regularly" cut fairways would liken trying to hit off them to hitting off a hardpan surface. Additionally with that firmness, sure if you hit it straight it can roll a mile, but there tends to be so many sidehills and undulations in the fairways of an open course that would eat an "average" golfer alive.
Secondly, the greens, sure their fast, but then again at just about every PGA Tour stop (and many, many private and/or higher end public course) there fast, but its once again alll the slopes and undulations that need to be seen in person to realize. NBC's 3-D green slope animation tries to depict this, but until you've seen the greens at eye level and see that there regular is 3 to 4 feet of elevation change from back to front or front to back and then lots of 1 to 2 foot high mounds in the green too, its really tough to appreciate how difficult they are.
Lastly, the rough, everyone talks about it, and its NASTY! We're talking 6 to 8" green steel wool basically. Literally when I was at Winged Foot last year, if I was standing at the gallery rope line I had about a half dozen times where someone drove the ball in the rough within 10 feet of where I was standing and even from my lofty 6'3" perch you just plain and simple couldn't see the ball
: Then the sound of the grass just grabbing the club as the pro was trying to escape, the best thing I can liken it it, is if you've ever played a course next to/near a marsh area and been close enough to the marsh when it "eats" a golf ball, that "thwap/slurp" sound is what it sounds like.
Also, course management. This is exactly why Tiger's comment about a 10 hdcp'er not breaking 100 I feel would be true. Until you get to a really, really high level (sub 5 handicapper in most cases), one's course management strategies are generally a weakness. The abilty to know your game and have it be consistant enough to hit the shots where you want consistantly is what plagues the majority of golfers. To successfully play an Open course, you really, really need to be able to first realize where you need to hit the ball (and sometimes that may be 30 feet AWAY from the hole) and then be able to execute that shot. An open course generally will have such a small target area that will accept a shot that a 10 handicapper will likely be spending most of their rounds hitting their second shots out of the edges of that "steel wool" rough, and then having to hack out of that greenside rough also because they didn't play to the receptive part of the green and/or they couldn't get enough spin on the ball to hold the green.
I almost guarentee that the 10 handicapper wouldn't break 100 if their weren't forcaddies/marshals in the driving landing area to help locate wayword drives. Without them, you trying to find your ball from 200+ yards away in that thick rough would probably add a minimum of 3 to 4 penalty strokes to your score in lost balls alone.