If you follow the challenger tour you'll see that the level of play is significantly slower than in the tour. Basically most consistent players have a lot to gain from collecting the remains in matches against guys that actually have weapons but are inconsistent or/and headcases.
Clay is clay, most challengers on clay are won by specialists, guys that have played their whole life on the surface.
And on the faster surfaces, big servers always win. In a way, the challenger tour is a bit of a mirror of the 90's when the range in playing styles from winner to winner of tournaments was drastically different.
It's pretty interesting.
Great points, the difference in overall level is stark. Also, the real talents, they always rise to the surface after a while. As we saw Grigor and Berankis do at the end of last year. Players that actually are ATP quality rarely are unsuccessful in a big way in challengers.