Here's my take on some of the differences between forums here. An issue that I come across often when I talk to teaching pros or other tennis fans is that most juniors and even many adult tennis fans haven't watched tennis before 2000 or so. That really skews perspective, since it's difficult to appreciate any player pre-Sampras or pre-Agassi. That's the case for people that actually play tennis. Imagine the narrow perspective of those that (a) started watching tennis sometime after 2000 and (b) have never played much competitive tennis. That's not the case for most any poster in the Former Pro section. I bet most anyone that regularly posts in the Former Pro section has (a) watched tennis from well before 2000 and (b) has at least played tennis for years, if not a lot of competitive tennis. I have watched and played tennis for about 36 years. I still play tennis about 3 times a week and cross train a lot as well, so tennis is really a lifetime sport for me. I get the sense that for MANY posters in the GPPD section they have hardly played any actual tennis matches. I think many of the problem posters in that section are fans of certain players, but they just don't have a lot of direct experience with the sport. That tends to create a lot of misunderstanding with a sport as nuanced as tennis.
Hum I think you understate the importance of humility and honesty. A lot of posters don't consider that they may be wrong and it allow them to hammer their certainty by using capitals, exclamation marks, histerical laugh and insulting words!!! Another big part of posters simply don't care with the issue of being right or wrong: they are fan of a player, they hate his rivals, they love to argue on a forum and display their excellent rethoric.
The number of such posters is awful in the current pro section to the point that half of the posts in this sections are written by people I have on my ignore list! (by the way, use this list, it's good for your sanity). But their is also a few posters like that here, unfortunately.
So as pointed by ARFED and others, having seen more tennis and being older is something which should help people to be more humble and more honest, but it doesn't work for everyone. For some posters, the last great tennismen played in the 70's. For some other, their is only Sampras. Some don't watch much current tennis too.
Finally a lot of tennis fan are now able to access a lot of information via youtube, wikipedia, the ATP site and a lot of other websites. For myself I began to watch tennis in the early 2000, but it was the WTA (Hingis). Then I followed the ATP but I hadn't access to watch a lot of tennis until the mid 2000. Nonetheless, I watched former matches on youtube. And believe it you don't watch a match in the same way when you are little emotionally involved in the outcomes. Example: The wimbledon 2008 final will probably remain the best matches I have seen, but I know that it wasn't one of the best match played. Yet I was emotionally involved in it which give a bias to my opinion on this match, and a bias against all the more recent matches people claim are the greatest in which I'm not as much involved (for the only reason that I don't accept that a greater match could exist that this one). For this reason I already now that in 30 years I will still ramble about this match, and dismiss the greats of the moment as necessarily inferior to the two titans I witnessed in my long lost youth.