One thing that many newbies may not be familiar with when it comes to tabletop RPGs is the role of the Narrator, or Game Master.
One of the main things that a Narrator does is create the impetus for adventure. When I was first researching Oz RPGs (to see if anyone else had a similar idea), my Google searches turned up a lot of topics on various forums that were "Let's start a Wizard of Oz RPG!" The first few posts are where characters are decided (who's the Scarecrow, who's the Tin Woodman, etc.), then the next few posts are everyone saying hello in character. That's when the thread just dies. Without a Narrator, there's no one to say "And then the Winged Monkeys come flying in!" or "Glinda needs your help!" leaving the characters with nothing to do but exchange silly pleasantries. Some of these RPGs are able to go on longer if the players are willing to be proactive and create situations for their characters, but this seems to be fairly rare.
One thing that I found interesting was how some Internet-based RPGs will have "Stealth Narrators." That is, the moderators or other people-in-charge will have their own characters, just like the other players, but these characters are the movers and shakers in the setting. In an Oz RPG, this would be characters like Glinda, Ozma, or the Wizard. This allows them to guide the story (or stories) much like a typical Narrator in a tabletop game.
And of course, one trick that the Internet allows for is the assumption of multiple identities. So a Narrator can be signed in as Ozma and tell a player "Go talk to Glinda." Then the Narrator signs in on the Glinda screen name and lets the player interact with Glinda.
One downside of this approach is what is known as the "GMPC." This is where the Game Master has their own Player Character embedded with the rest of the party. Some GMs do this with the best of intentions, having someone there to provide a little boost to the group if they need a hint or a little backup. And since supporting cast characters are not limited to the same level/ number of points/etc. as the other players, this GMPC often winds up being more powerful than the other characters in the party. Often, the game devolves into "The GMPC Show" with this character providing most of the story potential and firepower, making the players feel like spectators instead of protagonists.
Another important function of the Narrator is the mediation of action. In a tabletop RPG, this involves handling usage of the rules as well as settling arguments and keeping everybody focused on the adventure. Because it is very possible for a discussion of the peace negotiations between the Flatheads and the Skeezers to derail into a debate about the physics of Robotech given the right group of people (All you gamers out there, tell me you haven't seen that happen).