Well, I will admit that if push came to shove, I would rank Nadal ahead of Kuerten, Wilander and Lendl, as well as Lacoste (though not yet Trabert and certainly not Cochet, Rosewall or Borg). Still, "head and shoulders" is an exaggeration.
My point is this: if Nadal retires tomorrow, history won't remember him as being much greater (if any) than any of the aforementioned players. The fact that he won his three French Open titles consecutively may give him a slight edge over Kuerten, Wilander and Lendl, but his legacy won't have anything to do with win streaks or minor tournament wins. These things are trivial and quickly forgotten, until the media decides to dig them back up the next time a current star makes an impressive run. At the end of the day a player's reputation stands or falls on the big wins--in this day and age, that means the majors--and for Nadal to put some real distance between himself and Kuerten, et al, he's going to have to continue his success at Roland Garros for at least another year. And he needs 2-3 more years before he should start to be mentioned in the same breath as Cochet, Rosewall and Borg.