A lot of RPGs have their own painstakingly detailed setting and core adventures which the players are intended to at least read through beforehand. I have never run a pre-generated encounter. To me, over half of the fun of GM-ing is world building. So using someone else’s world is just not as engaging to me. On top of that, using a premade world usually means I have to learn about that world. I tend to make up the characters, plots, encounters, and most other things on the spot tailored to what the group seems like they want to do. Doing that in a set world always concerns me, because I don’t want to contradict an established piece of lore, or character. I use general settings as a basis for the stories I tell, but usually change what appears on the map.The players may be on a different continent than any that are listed in the book.
Everything I just mentioned falls under the setting of a game. Separating the rules from the setting can be fairly difficult, or incredibly easy. In the case of Dungeons and Dragons or Pathfinder, as long as the world you make is in a fantasy-esq setting then you are pretty much good to go. What if you wanted to run a modern era game with the Pathfinder rules? That is also fairly easy. Make up the stats for guns and other technology, decide if magic is present, then limit some of the gear (walking around in a full plate suit of armor is not normal in the modern era… usually). Dungeons and Dragons usually has its rules slightly separated from the setting, at least enough so that it is easy to fudge them to fit into a new world.
The interesting challenges come up when you’re dealing with a game where the setting influences nearly every rule used. Eclipse Phase, Shadowrun, and (to a lesser extent) Numenera come to mind for me. In the first two games the setting is a post human society either with technology (Eclipse Phase) or magic and technology (Shadowrun) having a huge impact on the world. In these cases, the rules are what I wanted to lose. The settings were ones which I actually wanted to make stories for.
Running an Eclipse Phase game in a custom setting is nearly impossible. One of the key features is that your mind can be separated from your body in the form of a digital back up (think Ghost in the Shell), when you die you wake up in a temporary body somewhere at the last time you backed up you memories. It becomes incredibly paranoia driven as you try to find out how much time you are missing and how exactly you died. The setting is one of the coolest I have ever read about, and just writing this now makes me want to play it again. The downside, because of course there’s one, I’m not a huge fan of the rules. Well, I nearly completely hate the rules. It uses a percentile system, something I already don’t like very much, which is varied in slight ways that I don’t fully understand. It’s essentially entirely my own fault I don’t like the rules, but still… percentile systems are lame! The creators have good heads on their shoulders, but my personal dislike of percentile systems makes this game not as appealing to me as others. Though I think I’m going to convert the setting to a D20 system simply because it’s so incredibly cool.
Shadowrun has a similar issue to me. The setting is also super incredibly cool. Essentially, technology has reached new heights and corporations own the world. On top of that, dragons returned to the world bringing Mana with them and re-infusing magic with the Earth. Technology and magic play off of each other in interesting ways throughout the world. You have some pretty cool interactions as a character may pilot a laser tank into a hail of gunfire, fireballs, and magic missiles trying to commit corporate espionage (gone wrong, in this case). Again, I love the setting, but the rules leave much to be desired. This is a dice pool system. Say you attempted to shoot a gun, for instance, you total up the value of the ‘shooting’ skill (we’ll say 4), the value of the ‘aiming’ stat (2, maybe he’s got crappy eyesight),and any additional modifiers (-1, it’s raining. Probably acid rain because it’s the future) to get the total for that skill (5 in this case). You roll that many d6s and total up the ‘hits’ you receive. Hits are usually 5s and 6s, though sometimes they can be 4s, 5s, and 6s. You then total up the ‘misses’ you roll, which are usually only 1s. Then the number of hits minus the number of misses determines if you are successful, and the degree of your success. This system is better than percentile to me, but it can be very punishing. it sucks having a total of over 10 and still rolling 8 1s. In a D20 system you usually only have a 1 in 20 chance of automatically failing (rolling a natural 1), so adding modifiers to your roll has a much more noticeable effect on the game.
The cypher system (used in Numenera and The Strange) is great, they have done a fabulous job making the rules just vague enough to enhance play more than hamper it. There are very few things which actually alter the D20 roll, instead you alter the number you have to match. As the difficulty decrease (because of training or having your character focus on the task) the target number decreases in turn. This makes each encounter much faster to make and run as you essentially pick a bunch of numbers instead of looking up stats and variant rules. In the cypher system character creation is streamlined into writing a sentence about your character. I won’t go in depth into what that means, but just know that it’s really super easy. Making different options for characters in the Cypher system is simple, just follow the power level given by the other options and try not to make the characters incredibly OP (but, again, OP in Numenera is kind of hard to determine). The more challenging part of adapting the rules in Numenera to a new world is all the equipment. It’s not so much a challenge as it is a disappointment. The little items that you can randomly find throughout the world are one of the more fun parts of the experience. Removing the setting from them takes away some of the character they have. Throughout the game characters are intended to find small objects, fittingly called Numenera, which usually have 1 and done effects. Grenades, pills, and single shot guns come up frequently, but sometimes you find something more interesting. Gloves that accelerate someone along a surface, a belt which prevents ALL metal from coming within 5 feet of you, and an emitter which gives everyone around it hallucinations are just the ones which I can remember off hand.
Some RPGs have great settings and poor rules, while some have cruddy settings with fabulous rules. Some have both! To me the setting is more important than the rules for a game. I like learning new rule sets and seeing what different designers come up with for common issues, but what’s most important to me is that the players are having fun. If they are engaged in the setting then, to me, its good enough. It is also important to note that nothing is going to be perfect. There is no ‘quintessential’ RPG world, just as there is no pinnacle of RPG rules. There doesn’t need to be though. The rules just need to be good enough and the setting needs to be interesting enough. The rest is up to the players and the GM.