Pendragon is the RPG of Arthurian Fantasy. I was introduced to it in my senior year, and enjoyed at the time, though in retrospect I didn't "get it". However, I did enjoy the game a lot, and have now taken the time to acquire the massive campaign supplement that lays out 80 years of campaign time, plus two additional chapters that are just adventures. It's an incredible work, and I've enjoyed reading it a great deal, even though I haven't read all of the scenarios, in the hope that I might someday encounter them as a player.
The other thing I've realized is that Pendragon is fundamentally different than other RPGs in a number of respects that make it very challenging for most roleplayers to handle the setting correctly. There are several of these, so I think I'll go through the ones that I find most interesting.
My particular problem with "getting it" came about because of religion. The game presents a very confessionally mixed nobility, and the other players certainly represented that. I played a zealot, and was at least indirectly antagonistic to everyone not of my particular confession almost all the time. This was a mistake not just because it created an uncomfortable play atmosphere, but because it inaccurately represented the relative secularism of the nobility in medieval times: even a faithful noble would not have behaved as I thought was appropriate. The hardening of confessional lines is much more a characteristic of early modern times.
A big stumbling block is gender roles. While the statistical difference between the genders is very minor, the games makes no bones about the fact that the material will be written under the assumption that traditional medieval gender roles are being observed. This means that in most situations women are in fact second class citizens. Needless to say, roleplaying this could easily make anyone with a more modern viewpoint uncomfortable.
Another thing, especially in the course of the great campaign, that might be challenging is the scope. Quite frankly, eighty years of game time is simply too much for any single knight or group of knights (and this is without the occasional notations that say things "this is important, so kill half the group"). The solution in the game is that the player has a family, which is a source of new knights via offspring or other relations. However, it requires that players maintain a very different attitude towards their characters: the emotional attachments to characters need to be lighter in a game where their death/retirement is expected. Furthermore, unless they consent to do otherwise or do something truly stupid, players will not all be cycling characters at the same time, creating power level mismatches within the group.
There's a complicated point I'd like to make here about families, legacies, and luck, but it's slipping away from me. I'll have to try to spell it out another time.
The last issue that I think is a major adjustment for many roleplayers (especially in the Exalted crowd) is that the players are normal people living their lives amongst living legends. Yes, the PCs are above average, espcially in the important realm of killing things. And yes, they should be the focus of the story: Arthur and company already have their stories well documented, after all. But the PCs will not be the equal of the great knights. For example, as written it is mechanically impossible to beat Lancelot in a fair joust. A PC will have to have extraordiary luck and skill even to tie him. While most players understand that there are forces in the world with greater personal potency than their characters, the thought that it is impossible to be better than a certain someone at several important skills (especially over such a long time) might be displeasing to many players.
I am still fascinated by the game despite all of that, and would love to play it, given the right group of players with the proper mindset and determination to see the thing through.