Ted:
Sorry to be Clintonesque, but to answer your question I have to go back to asking what you mean by "difficult for the Tour players".
If you want to make it difficult for a guy ever to shoot 62, you make the greens as difficult as Oakmont's. And if it's soft, I'm betting there's somebody out there who is one shot better than Johnny Miller, 33 years on.
My comment about the answer being rough and trees was serious, if the question is do you want the qualifying score to be as high as possible. Tour pros are not the straightest of hitters from week to week, and rough and trees will affect more than half of them. I'm not saying that is the way to INTERESTING golf, but if you want high scores, that's the way to get them. It won't prevent somebody from hitting every fairway and shooting 62, but they have to hit almost every fairway to do it. For a demonstration, just wait til the TPC at Sawgrass in 2007!
Length, as you say, is not as important. Unless the course is OVER 8,000 yards it isn't going to be hard for Tour players.
The single toughest thing for Tour players is wind, but that's only a consistent factor in certain locales.