When I compared Oz to Star Trek a couple months ago, there was one very important thing I left out. I didn't realize what it was until I read this Grognardia blog (Thanks for the link, Groknard).
One of the things that captured the imaginations of millions for both Oz and Star Trek was the lack of continuity. You didn't have to follow the series to enjoy it. You could watch any episode or read any story in any order and be able to enjoy them.
If there was something you needed to know from a previous installment, you were told very quickly or it was simply glossed over. The Patchwork Girl's origin story was not rehashed in every story she appeared in. She would simply appear, perhaps with a small explanatory note, and the story would proceed.
Later Trek shows built on these original stories, but lost this simple charm. They developed a more stringent continuity, requiring the viewer to follow the show more closely. Deep Space Nine in later seasons became very soap-opera-ish in terms of continuity.
Oz fan writers often do much the same thing. They will write "sequel" or "follow-up" stories that refer to something from the old books and assume that the reader already knows what's going on. While I am not going to say that all such "sequels" are bad, I will say that they are rather difficult to do well.
How does this apply to gaming? Well, back in the day, RPGs followed a very similar format. Not by design, but by a lack of design. Early roleplayers basically treated their characters like golf clubs, choosing one to play based on the needs of the current dungeon. "4th level? You guys need a priest? Got one right here."
Much like television, gaming has expanded to include ongoing campaigns with elaborate storylines and a cast of recurring characters. Also like television, this serves to separate those who got in from the beginning and "get it" from those who are just starting out, trying to get involved later on. This has the side effect of making roleplaying very intimidating to new players. If you haven't been playing for several years or have the latest edition of the rulebooks and know the game world inside and out, there are people who will not give you the time of day. The books themselves have become larger and more expensive, a far cry from the simple booklets of yesteryear.