There's such a thing as VARIATION.
I think people just like complaining.
Here are the return numbers from 1991 for the best returner in the game on each surface (over 100 return games):
Hard - 36% (Michael Chang)
Clay - 45% (Andrei Chesnokov)
Grass - 33% (Stefan Edberg)
And here are the numbers for 2011:
Hard - 41% (Novak Djokovic)
Clay - 44% (Rafael Nadal)
Grass - 31% (Novak Djokovic)
Clay and Grass are almost identical. There's a big variance in hard courts, but that was an anomaly as Novak was having an outrageously good year. Number two was Murray with 37% (and that was Novak's number in 2012 when he was no longer a super hero).
So where is the complaint about variety? Grass is a faster surface - the serve means more there Net play matters. Clay is slower, easier to break - long rallies, baseline game, topspin rules. Hard courts are inbetween.