Just to define some terms here, there’s Law, which delineates the boundaries between acceptable and unacceptable behaviors, defines the powers and obligations of government and defines the rights and obligations of the citizenry.
Then there’s law enforcement, which is about busting violators and preventing infractions.
Then there’s Justice, which interprets how the law is practically applied, including resolving disputes and enacting penalties.
So if we’re just talking here about law, as opposed to its practical application, then I’d say that the line between acceptable and unacceptable behaviors should be determined on the basis of preventing harm to the security, property and health of others (I say this, though, knowing full-well that the question of what exactly constitutes “harm” is a murky one; I don’t, for instance, know exactly how one goes about resolving questions of shielding children from sexually explicit material. It’s sometimes difficult to untangle our tradition- and religion-based notions of harm from real evidence-based determinations; are boobs “sexually explicit”?).
It should also be crafted to ensure equal standing of all members of society before the law (but even here, how do you determine when a person loses some of that standing, and how much of it he loses? Murky again). This includes making accommodations for the disadvantaged (murky again).
And wow, defining the powers and obligations of government and the rights and obligations of the citizenry—I don’t have enough time remaining in the year to delve into that, but I’ll just say that Americans have thus far been a bit too timid about bringing the power of government to bear on problems of human suffering. As tools go, government is a big hammer, capable of doing lots of damage when used indiscriminately. But when there are nails to be driven to build a decent life for all our people, then we just have to swing that hammer as best we can.