What makes a rule enforceable?

The board needs real authority to make this particular rule

A board's power to make rules is delegated, not automatic. Getting elected does not by itself grant power to regulate anything the board can think of. The authority has to trace back to the declaration, the bylaws, or a state statute.

"[A]n association enjoys an implied power to make rules in furtherance of its power over the common property," but "has no inherent power to regulate use of the individually owned properties in the community."

Source: Restatement (Third) of Property: Servitudes, section 6.7, as quoted in Moretto v. Elk Point Country Club Homeowners Association, American Law Institute

This matters most for rules that reach onto an individual lot: paint colors, landscaping, what can sit in a driveway overnight. A rule restricting the structures, landscaping, or design of an individually owned lot needs a specific grant of authority in the declaration or a statute, not just the board's general sense that it ought to be able to regulate the community.

The rule has to be reasonable, not just approved

A rule the board adopts is not judged by the same standard as a restriction written into the declaration. The declaration carries a strong presumption of validity. A board-adopted rule has to clear an actual test: is it reasonable? Exactly how strictly courts apply that test varies by state; check how your own state's courts have treated challenges to board-adopted rules.

"[T]he rule of reasonableness" is "the touchstone by which the validity of a condominium association's actions should be measured," and an association "is not at liberty to adopt arbitrary or capricious rules bearing no relationship to the health, happiness and enjoyment of life of the various unit owners."

Source: Hidden Harbour Estates, Inc. v. Norman, Florida District Court of Appeal

Owner approval does not settle the question by itself. In one case, owners approved a challenged rule by a two-to-one vote, and the court still went on to evaluate whether the rule itself was reasonable. A popular rule can still be an unreasonable one. And the conduct being regulated does not need to rise to the level of a legal nuisance before a board has grounds to address it; the bar for a rule is lower than the bar for a lawsuit.

It has to be applied the same way, every time

A rule that is reasonable on paper can still fail in practice if the board does not apply it consistently. Consistent application, documented reasons, and even-handed decisions are what make a rule defensible once it is challenged. See Consistent enforcement and Selective enforcement for what happens when a board enforces a rule against one owner but lets another slide.