As society's ideas of what is unacceptable behavior changes, crimes also change over time. Things that were once crimes (like adultery) are no longer criminal. Others have become crimes only recently (heroin was legal as recently as 1925). Some crimes still on the books are no longer actively enforced, such as cattle-rustling.
This is desirable; as society changes its perceptions about not only what's right and wrong, but how serious a violation is, laws should change as well. It suggests that the criminal laws change in response to society's views. This is in contrast to civil laws, where often the law precedes the attitude adjustment of the society (Brown v. Bd. of Education, for example) and even when it's been made clear, society and its governing bodies often drag their feet (Brown I, the first time the case came up at the U.S. Supreme Court, was originally decided in 1954. The 10th U. S. Circuit Court of Appeals noted in Brown v. Bd. of Education III that vestiges of segregation persisted in Topeka schools in 1989, 35 years after Brown I was originally decided).
As to what's wrong and what's right, society has always had some ideas. The old common law felonies (easily remembered by the mnemonic "Mr. & Mrs. Lamb") were few and to the point:
The distinguishing characteristic between a felony and a misdemeanor was, of course, that a misdemeanor was punishable by mutilation, exile, fine or imprisonment, and a felony was punishable by death. It's interesting to note that acts that once were criminal, no longer are and we now prohibit things that were legal in the past. There are numerous examples of this from the laws passed in the wake of the 9/11 attacks. Often these laws are written to address what is a current politically-sensitive issue.
In the early 1990's, there were several violent car-jackings in Maryland, one resulting in the death of a woman trying to rescue her infant child from the back seat of the stolen car. The car-jackings received a great deal of media attention and the State legislators were inundated with calls for them to "do something". Legislators don't patrol the streets (thank God), so they responded with bold legislative action...and made car-jacking a crime.
Obviously, this didn't make car-jacking any less appealing to the car-jackers. No bad guys read the news and declared "Wow, I'm not gonna do THAT anymore!". The act of car-jacking was already several crimes; it required assault, sometimes battery, car theft, robbery, maybe armed robbery and other acts that were already criminalized. The public, however, was satisfied with these "tougher" laws, and legislators could claim they had done something.
We still struggle with why some things are illegal and others not. Some are historical issues -- alcohol was (almost) always legal, but so were drugs. But once we decided that all drugs are bad (so, don't do drugs, m'kay?), why did we criminalize certain kinds of drugs more severely than others? Why did the Maryland legislature decide to criminalize carjacking, when we already had laws against the behavior and the act? Can it just be to cover legislator's butts?
What do you think? Is there a crime currently on the books that you believe shouldn't be there? Keep in mind that there are lots of old dumb laws still on the books, but not all of them are crimes. A crime almost always states whether it is a felony or a misdemeanor and proscribes a sanction, either fine or imprisonment. If there's no sanction, it's not a crime. With that in mind, find a crime that you believe society has outgrown. One condition -- it can't be a drug crime, including alcohol.