Instead of playing games I now spend a good part of my weekends creating them. Or rather creating encounters, adventures, and a campaign for my Dungeons & Dragons 4th Edition players. Of course it is a lot easier to create adventures for 6 players than for hundreds of thousands or even millions. But some basic principles remain the same, whatever the number of players, and whether you are creating encounters for a pen & paper game or for a MMORPG.
One principal problem is how the players find the adventure. Realistically, if you walk into the middle of the forest, chances are that nothing happens, even in a fantasy world. If there is a clearing where druids holding a secret ritual, the players would first need to hear about that. And then they'd still need to find it. And realistically speaking, 99 times out of 100 the clearing would be empty once found, unless the players also know the time of the secret ritual.
MMORPGs and pen & paper games have similar solutions for this problem. In a MMORPG the druids will simply ALWAYS be on that clearing. And the "speak with the druids at the clearing" quest will have that clearing marked on the player's map, so finding it is no problem. In a pen & paper game the players don't need to know the exact location of the clearing, as "space" and "time" are more flexible here. If the players say "we search the forest for the clearing", the DM can roll some dice and declare that it took them 3 hours to find it, but those 3 hours just last seconds in reality. If the players then wait for the secret ritual to start, that again only takes a few seconds of "fast forward" time acceleration.
One trick both MMORPGs and pen & paper games use is scripting: The moment the players meet the caravan just HAPPENS to be the moment that caravan is attacked by goblins. In a MMORPG the script can be launched by the players turning up at the location, or, more frequently, by talking to a quest giver at the caravan. Pen & paper games use two different triggers: Site-based and event-based. Site-based scripts start when players turn up at a location. The classic example is opening a door in a dungeon, which starts the combat against its inhabitants. One of the classic D&D adventures, I think it was the Temple of Elemental Evil, had a 3 x 3 squares room with 15 orcs in it, but no mention of furniture or what the 15 orcs were doing in this small room all the time while waiting for the players to open the door. Dungeons in MMORPGs haven't really evolved much beyond that, monsters are mostly fixed at one location, with the exception of a few patrols. Unless the players do something extremely stupid, the monsters of one location don't interact with the monsters next door.
Where pen & paper games offer somewhat more than MMORPGs is with event-based adventures: Scripts that are triggered by time passing or situations evolving in a logical manner. The players arrive in town 3 days before the town fair, and certain events will happen during that fair. That works well in pen & paper, because if the players don't need time to do something before the fair, they can always declare that they wait 3 days and thus quickly advance time. The only equivalent in a MMORPG would be a holiday event, but these are never essential to the story. MMORPG players can't advance time as in a pen & paper game, so "something important happens in 3 days" doesn't work well in MMORPGs. Furthermore pen & paper games freeze in time while the players aren't playing, while MMORPGs continue, so what if the player isn't logged on in 3 days and misses the event?
A further advantage of pen & paper event-based scripts is that they allow players real decisions, or real consequences of their actions. In a MMORPG most battles you'll see are frozen in time and will never change, or just through a major content patch. In a pen & paper game the players can influence which side wins a battle or even a war. They could even have a diplomacy adventure that starts or ends a war. If they expose the major of a city as a secret demon-worshiper, that will change how the citizens of the city react to them. These are all things that MMORPGs have great difficulties with.
As Dungeon Master the main difficulty is how to create a good adventure with a mix of site-based and event-based encounters. I'm currently "redecorating" a dungeon which is completely site-based, because I don't like the "door-monster-treasure" structure. So I'm removing many encounters that don't really add to the story, and make sure that there are more events, dialogues, and decisions for the players. Fantasy adventures are never completely logical, but I'll try my best to keep up the suspension of disbelief by having the various encounters link up by a logical story, and not just a corridor and a door.