I guess the reasons I would give for the Open Era being tougher than the pro tour are:
a). To win an Open slam, you usually have to beat some lesser players to begin with, followed by the top opponents in the QF/SF/F - whereas in the pro ranks, you only had to do the latter. Yes, the average level per match would be higher in the pros. However the number of obstacles each player had to face would be lower - because they would 'only' be facing their fellow greats, not the 'lesser' players beforehand who, on any given day, could cause an upset (i.e. Rosol over Nadal).
b). Sort of related to the above is the issue of the head-to-head pro tours. It's often stated that Federer is greater than Nadal, despite his losing h2h, because of his greater accomplishments. Yet when posters talk about the old pro tour, the h2h's between the top players become of the utmost importance. The fallacy behind this can be shown because, if there had been a pro tour over the past 10 years, Nadal would have come out with the best h2h's against all his rivals. Yet we know, when playing the wider field, that Federer is greater.
Hence the h2h pro tours are not as meaningful as some make them out to be.
I would add, regarding the H2H, that Nadal wouldn't lead the H2H against Djokovic, Davydenko, Federer, had the the majority of the matches been played on indoor, especially on wood...H2H is very dependent of the surfaces on which most of the matches occurs. This another grief against the H2H things. What would have been the H2H of Gonzales if he had played the majority of his matches against Rosewall or Laver: you know it: he would trail.