I think Nadal dominated a weak(er) clay era, but as everybody else has said, most the time the clay era is weak. This doesn't take away anything from Nadal's wins, he just didn't have many consistent threats before 2008.
The same can be said for Federer from 2006-2007, as Hewitt, Safin, Agassi and to some degree, Roddick had dropped off the radar.
"Leavin' on a southern train only yesterday, you lied. Promises of what I seemed to be, only watched the time go by."