Roddick himself even admitted he's not good on clay.
He's not good on clay, but he's a great competitor and thus he can hold his own on the stuff and not lose miserably, even if it's against Nadal. Roddick put in a good performance considering (a) he was playing Nadal on clay in Spain in a bull ring, (b) the courts had been soaked by rain before the match, and (c) Nadal was playing, in his own words, "almost perfect"
He certainly put in a better fight than Federer did in the RG final, that's for sure. Second set was a wash, Roddick showed his inexperience as a serve and volleyer by getting down on himself from being passed so much, but 1st and 3rd set were quite interesting and Andy held his own. At the tail end of the match he hit the ball as hard as he could, with the kind of power the young Roddick used to show, and Nadal just got everything back. He got the balls back shallow, but he still got them back and forced the error from Roddick.