Bill,
Yes, weather and setup primarily. Less often, turf conditions (at times too wet as opposed to too firm, though firm, windy conditions along with high rough and an abundance of hazards may actually lead to higher scores).
Shivas,
The Kepler was/is indeed in April. You probably weren't born yet the year that I was thinking about, so you were not among those whom I was watching. With a 30+ mph wind, fairly cold, and the course as hard as a rock, holes like the par 3 #8 over water were nearly unplayable. That day, in the mid 1970s, Scarlet was about the hardest course in the world.
Huck,
I think that Carnoustie is world-class. It is very difficult, and I can only imagine what it must be like when the wind really blows. I am not sure that I would want a steady diet of it, but it seemed to me to be a great place to develop a game. What they did to the setup for the Open, in my opinion, was a real injustice.
This is off topic, but I thought that Vander Weighe (sp- the very nice French pro) played a reasonable second shot to 18 and got a very, very unlucky bounce. As nervous as he probably was, and after making the mistake off the tee and being the beneficiary of divine intervention, hitting the ball as hard as he could and clearing all the hazards made some sense to me. I could see him playing back to the fairway, hitting it too soft or hard and end up in the rough, and from there it is just a potluck.
Golf can just be a very cruel game which, within a 15 minute span, can take some of the best from euphoria to inescapable depression. I still remember watching Ed Sneed lose three strokes and his insurmountable lead at the Masters on the greens at the final holes. If he hits an imperfection on one green out of three which directs his putt towards the center of the hole as opposed to away from it, who knows where his career might have gone? If JVW's second shot hit somebody on the bleachers, or just bounced any way but backwards, where would he be today? I am not a big Dan Jenkins fan, but he probably has the game pegged about right.