Outdated, go here.
Rule: a principle or regulation governing conduct, action, procedure, arrangement, etc.
Rule: the customary or normal circumstance, occurrence, manner, practice, quality, etc.
Rule: a prescribed mathematical method for performing a calculation or solving a problem.
Rule: the constellation Norma.
When most people say "rules", they seem to immediately think "laws". The two are different words for a reason: the word "rules" includes things that aren't laws, such as most of those listed above.
The first one is the important one. "A principle or regulation governing conduct, action, procedure, arrangement, etc."
That's what a rule is.
A rule set is a set of principles or regulations governing any and all of those things. Which is dramatically more than simply "gameplay".
Sorry if I was unclear. I wasn't trying to waffle: I didn't think everyone thought "rule = gameplay". I mean rule as it is actually defined. Obviously, we're talking about the constellation Norma.
A game might have a "time-travel driven plot". People don't think of this as a "rule" because it's implicit rather than explicit. But it is: implicit rules still exist. A time-travel schtick is a principle that regulates the procedure and arrangement of the game's plot (along with other facets of the game).
It is a rule. So is the standard "scrappy youths save the world" schtick. They provide you with various guidelines - principles - which shape your design.
Things that aren't a rule are things like the dialogue you actually write, the characters you design, the specific cities you create, the specific subplots... these are things which result FROM the rules. They are also the things that most people think make a game good.