Well, I'm not so sure...
Medinah's par 4s go: 434, 414, 463, 474, 435, 438, 471, 392, 453, 443.
At Pinehurst #2 at the 2005 US Open, par 4s went 401, 469, 336 (the outlier!), 472, 404, 467, 476, 449, 378, 468, 492, 442.
I don't see a whole lot of difference there. Medinah winner likely to go lower than -10; Campbell was even par to win at Pinehurst, with Tiger runner-up at +2. And I don't get the impression, based on player comments at the time, that hitting fairways at Pinehurst was terribly challenging.
So I'm not sure variety is the key to more challenge. My sense is that course set-up, some key course architectural features, and the luck-of-the-draw weather conditions play a larger role. Pinehurst was set up firm and fast, and held that way due to absence of rain softening it up. And I clearly get the impression Pinehurt's upturned saucer-cup greens are much tougher to hold than the ones so far at Medinah. Maybe it's an apples and oranges (PGA vs. USGA) comparison. But I don't get the sense that variety in and of itself is the key to more challenging scoring. It's clearly more fun to watch, in my view, but in going over major tournament scoring the past decade or so, it seems largely determined by course set-up, weather and key architectural features (tough greens, narrow fairways, others...)