Facts that are hardly ever mentioned in tennis forums
1. Bjorn Borg won several of his French open titles during a time when one of the best players wasn't allowed to compete.
Borg won his first French Open title in 1974. The other remaining majors that year were all won by Jimmy Connors. Guess who wasn't allowed to compete in the French Open that year since he was a member of the 'wrong' tennis organization? That's right, Jimmy Connors. In fact, Connors didn't play in the French open in any of the five years between 1974 and 1978 in which his year-end ranking was number one, and when he finally returned to Roland Garros in 1979 he managed to reach the semifinals there (which he also achieved on three other occasions). In addition, Connors actually defeated Borg twice in the US Open in the years 1975-6 when it was held on clay. It should also be noted that Borg skipped the French Open in 1977 in order to play for world teamtennis. Can you imagine any top player doing that today?
And yet, whenever the discussion of greatest clay court player is discussed on tennis forums, this fact is conveniently omitted and people say Borg is better than Nadal since 6>4. Can you imagine how many US open titles Nadal would have if it was held on clay at some point during his career?
2. Not all the top players showed up for The Australian Open and the French Open in the past.
As mentioned before, Connors wasn't allowed to play in the French Open for several years. He also only showed up twice for the Australian Open (and achieved strong results by winning once and being the runner up the second time). Borg only played once in the Australian Open.
Thus, one cannot really compare the number of majors won by players from different eras. By winning 16 majors, Federer has become the most accomplished tennis player, however noone can really say how many majors Borg and Connors would have won had they regularly participated in the Australian Open and in Roland Garros (in Connors' case).
3. It was harder until recent years to win majors since you could meet top 30 players in the first two rounds.
Part of the reason why many top players in the past lost in early rounds of majors was due to the fact that there were only 16 seeds and not 32 like today, and thus they could lose to a top 30 player in the first round. Notable examples of this include Richard Krajicek losing in the 1996 US Open (after he won Wimbledon) to then top 30 player Stefan Edberg in the first round, Thomas Muster losing to then top 25 player Tim Henman in the first round of the 1997 US Open, defending champion Boris Becker losing to then top 25 player Carlos Moya in the first round of the 1997 Australian Open and Jonas Bjorkman losing to then top 25 player (and former champion) Thomas Muster in the first round of the 1998 Roland Garros.
Even if the top player does manage to win his match against his top 30 opponent, it might require many hours (and sets) to do so, and thus it might lower his chances of advancing to the latter stages. A fine example of this is then number 1 Pete Sampas' 1996 French Open run, which included a long second round match win over former 2-time winner (and then top 25 player) Sergi Bruguera and a third round win over then top 20 player Todd Martin. Under today's rules he would also meet a lower ranked opponent in his third round.
Nowadays, since players do not face top 30 players in the first two rounds, it is usually easier for them to at least reach the third round and find their rhythm and the tennis form needed to advance to the latter stages of the majors. This is why it is slightly unfair to compare today's players achievements in majors with past players' achievements.