Whether you intend it or not, when you create rules for a game, you are also creating the setting. Sure, you're not naming NPCs or populating dungeons, but you are establishing tone and the underlying logic of the setting. By creating a rule, you are saying "things work roughly this way" and player expectations are going to be set by this. If the rules say one thing but the world works another way, the seams show in your game and it suffers as a result.
While you can sidestep this with very meta rules (which have their own benefits and drawbacks) if you're rules are supposed to represent what happens in the game then you're stuck with this reality. And that's ok. You just don't want to accidentally design a rule that says something very different about your game than you intend.
There is a small rules decision which is often overlooked as incidental, but which has a HUGE impact on the implied setting of a game, and that is the effectiveness of ganging up. Why is this important? Because the power of groups is an important part of power dynamics. It is what makes things like gangs, guards, soldiers, policemen and mobs very dangerous. An individual might or might not be badass, but five or twenty of them is something to worry about.
Now, I'm not attempting to speak to "realism" here. In the real world, ganging up on someone is effective, but attempting to measure how effective is just begging for a nerdfight. Instead, I just want to discuss it in terms of the impact on the game.
The number of people one person can fight in a game is, of course, a function of their respective capability. An action movie star fighting schlubs shoudl be able tot ake on more of them than he would highly trained adversaries, and he'd presumably be hard pressed to fight someone in his own "weight class" as it were. This relative advantage points to an important mechanical point - how big is the range, mechanically, between the most badass guy and the least? That range is the spread ganging up needs to be able to cross.
This is, for example, why ganging up doesn't work in 4e, at least in the abstract. If I have a 30th level fighter, it really doesn't matter how many level 1 guys you run at me. Even if by some brilliant combination of luck and tactics you start to actually hurt me, I can pretty much just walk away and come back with an atomic bomb, If we're within a given level range than ganging up works fine, but while that may be a fair assumption for an adventure, it's a poor one for the world. It also introduces weird incongruities, where the level 7 sheriff of a town is more dangerous than everyone else in town put together.
Practically speaking, you've got four rough ways to handle ganging up.
Strong groups - Just adding one or two new opponents is very dangerous. Mechanically, this is usually represented by an active defense or the like, where you get your "good" defense against the first attack, but then either no defense (or a rapidly degrading defense) against subsequent attacks. In a system like this, numbers are more important than range of power, and this is probably the grittiest way to run it because in this situation fights are going to be dangerous.
Dramatic Groups - This is probably the default of adventure fiction. A small number of less skilled opponents needs to be taken seriously but is not too dangerous. Think Indiana Jones for a yardstick. Usually this is mechanically represented by granting bonuses for outnumber, flanking, surrounding or the like. The advantage of numbers is real, but its comparable to other advantages like weapons or position.
Decorative Groups - This is where you start seeing rules for mooks or extras - large numbers of foes easily cut down by the badassness of the heroes. In fact, part of that badassness is the ease with which they dispose of these throngs of enemies. Named and noteworthy enemies may still be a challenge, and they may derive some benefit from being surrounded by minions, but not a lot.
Meaningless Groups - Curiously, this is roughly similar to Decorative groups in terms of potency, but in the absence of rules to handle the groups gracefully. An example of this would be a D&D game where I run a lot of low level monsters at you rather than minions (which have 1 hit point). The low level monsters aren't a real threat, but now I need to track their hit points and otherwise do fairly dull bookkeeping for no real reward.
It is possible to mix and match these, even within the same system, if players are comfortable with the idea. 4e, for example, is mostly a middling groups game (within level range) but has minion rules to bring in decorative groups, and striking a balance between those two approaches is a bit of an art.
So what do these things say about the world? Well, for example, what happens when you try to arrest a character? In a strong groups setting, a handful of patrolmen can probably do the job. With strong groups you probably want to send the equivalent of a SWAT team. With decorative or meaningless groups, there's a good chance you just can't send enough men to do it.
That's simple enough, but what does that imply about the setting. Can powerful characters and NPCs run amok? Are there other powerful characters waiting in the wings to deal with those people, and if so, who watches the watchmen? How do kings stay in power? Are they high level badasses? What about their kids?
Now, it's important to note that the fact that these points raise questions does NOT mean that you can't come up with answers for them (and, in fact, you should). As I said before, we're not talking about realism here, so don't knock yourself out trying to determine the impact on global power dynamics of a 24th level wizard. Just look at things in broad strokes and tune your game to suit.
For example, while I hold up 4e as an example of a problem (because of the level range) there's an implicit assumption in the game that as you move into higher levels (or tiers) you are changing your context, moving from mundane matters to the otherworldly and eventually the cosmic. The town guards in your hometown may be people you can defeat with a sneeze now, but in the City of Brass, the streets are patrolled by fire elementals, and they're tough enough to take seriously. This is a bit of a combination of a treadmill and "you must be at least this tall to enjoy this part of the setting" and is probably very familiar to MMO players
Anyway, there's no correct way to handle ganging up, but I wanted to unpack it a little bit just to illustrate how one simple rule can say so much about a game.
1 - As opposed to rules that represent how players should act, such as who is telling a story at any given moment. When I talk about very meta rules, means ones which are more about the act of creating the fiction of the game. Despite a great deal of hyperbole, this is not always a clear-cut distnction, and games played as well as game designs exist on a it of a sliding scale in this regard. This is too big a topic to be just a footnote, but a good yardstick for distinction is this: does it matter, mechanically, whether you brought a dagger or a flamethower? For purposes of this post, I'm assuming the answer is "yes".
2 - And other modifiers too. If you ever watched the hold Highlander show a gimmick they would use a lot to maintain the protaganist's badass status was to introduce enemies who were super badass under some specific circumstance, like fighting in the dark or because they gassed their enemies before fighting. represented in game terms, that allowed a lower skill opponent to close the gap with bonuses and penalties.
3 - That awkwardness is why a lot of broad power range game eventually ended up adding mook rules. Decorative groups may be a bit of a narrative convenience (though they are brilliantly described as a setting element in Scion) but they are so much smoother in play that people are willing to be forgiving.