The function of a rule is to simulate the reality of the game-world.
It can never do that perfectly, but it should have just enough
complexity in it to do the job well enough without getting too
cumbersome.
By realism I mean simulating things which should happen in
the gameworld: this isn't necessarily the same as things that happen
in the real world. So in a superhero game, the rules would cover
superpowers that exist in the gameworld but not in the real world.
A rule can never be completely realistic. After a certain point,
as you add more complexity to it, you start creating more unrealism
with everything you fix. In particular, the more complicated you make
the rules for something, the more potential you create for rules
lawyers to find something to exploit
(see rule 3).
But most of the time, rules can be made pretty simple and still
work well much of the time. (This isn't always true though - my
personal nightmare is writing rules for automatic weapons combat*.)
But often, there could be several simple ways of writing a rule
for something, and game writers don't choose the most realistic of
several simple alternatives - instead they choose a still simple but
less realistic option. For instance, systems like Basic Roleplaying
(Call of Cthulhu/RuneQuest) have simple to hit rolls which often
lead to unrealistic combat (see
here
for a discussion of this). Rolling two people's combat skills against
eachother, and having the winner hit the loser, would be as simple
but would be far more realistic.
Even worse is when rules are complex yet still unrealistic.
Middle Earth RP has extremely complicated combat rules that can
take a long time to resolve, with much flipping through the rulebook
to consult several tables - yet it often produces nonsensical
combat results. This is the worst of both worlds.
Here are some examples of rules that don't work well.
In Call of Cthulhu, getting a better weapon can actually
make your chance of hitting decrease: an untrained character has a
50% base chance to hit with a punch, but only 20% with a baseball
bat. So two equally matched, untrained fighters are having a fist-fight
and hitting each other 50% of the time. Then one sees a baseball bat
kying nearby, and picks it up. Now he's got a better weapon giving him
a longer reach than his opponent, yet his chance of hitting has dropped.
This is sheer lunacy.
Paranoia (1st edition) has no concept that one weapon is inherently
more likely
to hit than another. Your base chance of hitting depends simply on the
number of points you have in the skill and your modifiers for primary
stats. So an average-statted person with 3 skill points in a certain
weapon skill has a base chance of 30% to hit with that weapon. Now,
a slugthrower (assault rifle) can be used on single shot or full auto
mode. Full auto fires more bullets, yet there is no increase in the
chance to hit. Do the writers not understand that firing more bullets
gives a greater chance of at least one hitting?
* Automatic weapons combat is a mess. A shooter can fire quickly
at multiple targets with a high chance of hitting with at least one
bullet on each target. Each hit on target stands a high chance of
taking the target out of the fight, so he can't fire back.
The way I'd like to do it is to have combat rounds about 5
seconds long and give each shooter a number of 'shooting points'
which he can split up between different targets. Then a simple
initiative roll would determine whose shooting is resolved first.
That's a simple system, but it falls down because it means the
first shooter (A) gets to do 5 seconds' worth of shooting. If he
doesn't take the second shooter (B) out of the fight, B can then
shoot back at A for the same 5 seconds. But in reality, they should
have been shooting at each other for 5 seconds, and either one could
have hit the other at any point during that 5s and the one that got
hit would probably have been taken out of the fight. Expand this to
include a two groups firing at each other and it just gets even worse.
The only way I can see around this is to have combat rounds
shorter than a second, and in each round allowing shots at only one
target, and giving them a fairly small chance of hitting. But I'm
not prepared to contemplate a game with combat rounds shorter than
a second - it would be worse than GURPS.