It's a very significant achievement. He should probably have won more majors by now except for his way of winning which is pretty hardcore and not exactly suited for longevity. Before anybody points out emphatically that "Rafa has been winning slams for 7 years, so there's your longevity, buddy!" I will mention that during the first three of those years, he was winning only on clay. By the fourth year, non-clay court speeds had been sufficiently slowed down by the powers that be in order to increase the likelihood of Rafa going deeper in non-clay tourneys, as Rafa's mom is keen to point out: "My son packs stadiums; he is good for tennis the business."
But the fact that he has a winning record over the big three is significant because it suggests that Rafa's peak level of play, given the state of the tennis courts on which this record was compiled, has been higher than that of the other three. Although, Rafa's evolution into an all-surface threat in slams went hand in hand with changes that were made to court surface speeds, as his first three years of winning slams on only clay--during a period when he was clearly faster and arguably hit more powerful forehands regularly, i.e., when he should have been even more imposing than he actually was--coinciding with the courts being deliberately slowed suggests.