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.
"I rest my case. Federer needs to win 28 slams, twice more than Nadal to be in the same conversation with Sampras." A famous quote from helloworld.